How do you get rid of slanderous attacks online by disgruntled employees?

That one is easy for me to answer.  Since I don't have any employees, I don't have that problem.  

I am no longer Santa Barbara location services please remove that and the word the word  is incompetence not in confidence 

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For one, solutions begin much earlier on the chain, in that what it sounds like, is that either the screening process failed, and that management didn't ask the right questions, or maybe when employees were hired, background checks were lackadaisical.  If you search social media (Ask employee if they're on SM of course) then poke around, most times you can get a pretty good idea of who people are.  
That aside, maybe you need to amend your employment agreement and specifically address this area in that document.  It should be initialed by the employee.  Now with 1099 (independent contractors) you can't go this route.  In fact your hands are tied up pretty well, as 1099 Indys are NOT considered "employees".  Many employers have the wrong idea about hiring independent contractors and putting them on a 1099 to save on taxes.  If for any reason you're unclear about the specifics of bringing aboard 1099 employees, and the differences with actual contracted employees, I suggest reviewing the laws in your state via Google.  They are very specific.  
That being said, I suggest having employees sign off on a doc and have it notarized, that states they will be prosecuted to the fullest extent of the law, if in fact they are responsible for slandering the company, or its employees in any way, and that any grievances will be handled in arbitration, not civil court.  By the way, I am NOT an attorney, nor am I providing legal advice.  I am also NOT an accountant, or giving you tax advice.  I am a business owner that has done the research and by doing so, have hopefully saved you a little time.  ;  ) 

Hope this helps! 

Every company has SOME negative feedback. Any exception is somewhat suspect. If the review says "Bad Service" or "Mean People", these are completely subjective. This is where good customers need to simply drown out the voices with positive, REAL, Reviews. If I see a 2 paragraph complaint, and 50 comments of "Great Place", I give the negative a much higher score. But if I see several positive comments that are a couple or three sentences, I generally disregard the negative.

OTOH, if the person is spamming a site like Yelp with negative reviews, then I'd get ahold of the site and explain what's going on. If they realize a single person is providing 10 negative reviews (and yes, they can track that), they can remove them all. If the negative is specific (x DID this, then DID that), then it becomes a legally actionable offense, and is regulated by law. 

I'm lucky in that all my clients are internal and my company's customers are corporations. So Yelp & Google don't mean much in my world.

I think that is a tough question.  If people know who you are they will know that what is being put out there is not you. Secondly, I would just be polite in my response and not get pulled into her/his web.  People will see other reviews and will most of the time read between the lines to see the truth.  Also, if that person is saying something that one of your customers feels the opposite about, ask them to write a review.  And in the future, think hard and long about whether to hire that person you are interviewing.  Call their references.

Ask advice from the local police office and obtain any brochure or information sheet they may have on that regard, anything printed. Then post that online in the platform where you get the attacks from. Include a general comment that you went to the police seeking advise on the issue, without mentioning any names, hopefully the person originating the attacks reads it, realizes you are not afraid to involve the police and escalate the issue forward and the attacks would hopefully cease.
Hope this helps. Wish you the best!

The problem is your question doesn't actually offer enough details for anyone to give you a sufficient response. What site(s) is this noted "disgruntled employee" leaving "slanderous attacks"?  If it's regular review sites where customers typically leave reviews, is he/she pretending to be a customer? Or using their real name and discussing why he/she is 'disgruntled'? Or is on Glassdoor or other sites where employees are given a voice to share their opinions about working for a particular company?

The answers to the above help to better identify what to do. Some you can remove, some you can respond to, others you have no action available to you... it depends on the site.  If they're not using their real name, you can identify in your response how you know it's this one person.  What the person is saying, also matters.  For example, if they're calling you 'scum of the earth'. You can "kindly" :) Respond, "I am so sorry to see this response. It is quite shocking. We would love to get to the bottom of whatever problem has caused this. Our number one goal is always customer service. Please call our offices so we can rectify whatever happened. Sincerely, said company."   

If they're stating that something particular has happened. "We are so sorry that we were not able to resolve your issues to your satisfaction. We are always working to create a solutions. We have not given up, please reach out to us directly so we may resolve this." 

If on employee site, you cannot talk about the employee's behavior or failures... simply state that "we are sorry that working with us has caused you such emotional harm. We do hope you can find a place of employment that better suits your needs and is rewarding for you."

All different responses to very different scenarios. 

It’s tough to answer. I would need more details. I would consult a lawyer since slander can be addressed through the courts. To protect myself I would be proactive and document my processes and situations along the way. Its a pia but it can certainly help if a situation such as yours arises. As far as negative feedback I’m a believer that you should respond professionally instead of ignoring it. 

Have your lawyer send a letter threatening Kesha’s action if they do not cease immediately and remove everything they have posted.

Respond to the online comment in a dignified and gracious way pointing out that you regret that the employee is using social media to attempt to injure the company's reputation.  Had they performed in an acceptable manner they would still have a job.