I'd hate to see it come to that, but I'm sure it would end if Litigation came into the picture. I'm sure if there's proof on the other party, then you could counter-sue.

EDIT:

Daniel, from what I've seen, some people haven't had their car done by them and are posting things not related to work done by them (correct me if I'm wrong guys)

Maybe "slander" was a bad choice, Defamation sounds more like it.