We have an 'A' frame sign stating the exact thing. Used it once and it worked like a charm.
Lawyers, courts and whatnot....... they're all a necessary evil. You see, the contract you had them sign, is one kind of law. It's contractual law. You do this, they do that and all are happy in the end. However, if you put the sign up, it's still considered theirs, just not yet paid for in full. However, if you have trouble collecting, that is civil law and it gets dirty sometimes. Now, if you go take your sign down for any reason..... even a bogus repair call tag, you are now tresspassing, stealing someone's property and committing felonies according to the signs worth. Those are not covered in your original contract.... are they ??
Just because you cannot figure out how to make someone pay for their sign, does not give you the right to commit so many illegal acts. You'll end up being in a lot more hot water then them with an outstanding invoice.
Sometimes the threat of removing it might work, but not usually. People are smarter today, than years ago.
There are laws to help you and protect you, but if you go about it wrong, those same laws will work for the other guy and totally against you.
Be careful and govern yourself accordingly.