We don't have any such signs in our shop. We politely tell the person we no longer work on dept. store bikes, and we give them the phone number and location of a newer shop owned by a guy that used to work with us, if they want it. Most of the time, they appreciate the info, and for the time being at least, our friend who owns the other shop is happy to get the business. As he becomes more established, he may decide he's busy enough with service work to install a similar policy as ours, I don't know.

Bottom line, there's no need to be rude or to post smart-ass signs IMO. You can deliver a message without offending people. Offend enough people and it's likely to hurt your business eventually.