Willful theft is rather disgusting and certainly reflects poorly on this person's character. You might post to their wall that you will be sending them an invoice at a special rate for stolen images and see what happens. (Failure to pay could lead to legal action based on the Copyright law of 1986 etc.)
Willful theft is considered by the courts under a much harsher light with stiffer penalties, however I would doubt that any real legal action would be worthwhile. The threat of action certainly gets the attention of individuals like this. If you have an attorney friend that will make a legal sounding letter for you to mail with the invoice that could be interesting and could scar the heck out of them. Mentioning federal law and potential damages and punitive actions up to whatever it is... 250,000 dollars ? per stolen image can be an eye opener.
Don't let someone's stupidity and lack of moral integrity control your life. If you are getting a knot in your stomach over this then that is not good. Rather, figure out how you can have some fun with this, maybe get some payment AND at the same time help get the information out there that is is not appropriate behavior. Make screen captures of your images and their comments and after posting to their wall, turn them in to Facebook and perhaps their account will be terminated as well. But, don't let others take control of your life.
I also don't think that making your customer experience crappy to prevent theft is a good plan. If you need to make a bolder copyright notice, go ahead. Personally I would not reduce the size of proofs too small so that the impact of the image is lost costing you sales. If you catch a shop lifter, do you then treat everyone who walks into your store as if they are shop lifters such that they no longer enjoy shopping at your store. No, you put appropriate protections in place and you keep an eye on things and when appropriate you take action. Do you get all knotted up inside? No, with a shop lifter, you just walk to the phone and call the police or security... you take appropriate action.