Well, you've certainly got a lot of responses, ranging from "copyright it" to "just share it".  Watermarking all of your advertising graphics is a definite yes (there are free and low cost programs to do this.)  Copyrighting is an idea, but (as mentioned) you can't copyright an idea - or there would only be one song titled "I Love You".  Plus, copyright is just a formal claim - it doesn't stop people from taking the product, it just gives you a legal right to challenge them.

Most people know Eli Whitney invented the cotton gin.  But they don't know he spent ALL of the profits on lawsuits for patent infringement (look it up). 

You don't mention what your business is, but if it's online you can do the following (some of which has been mentioned):

- Contact all your vendors and customers, and let them know you are the "original".  Make a point that a business that steals is not a good business to work with.  Don't mention names, people will know what you're referring to.

- If there is provable theft, contact the web host for that business and file a complaint.  Chances are, this person is doing other underhanded things.

- As mentioned, inject yourself into the process.  YOU can't be duplicated.  We think of "Flo" as the face of Progressive Insurance.  Become an integral part of your business

- If possible, see if you can blacklist the quote requests coming from his IP address or his account.  I don't know how you're set up, so you may want to ask your web host about this.

- DEFINITELY send a "cease and desist" letter.  Crooks are generally not that bright.  It may be enough to scare him/her, and DEFINITELY follow up those letters if they don't stop.  They may think it's just not worth the hassle.

Good luck, and I'm sorry you're dealing with this.  On the bright side, this person may not stay in business for long if they're that inept.