I think the site should have a rule - 3 comments that are lowball and insulting and you are off the site - paid or free - and the reading of that one condition made mandatory as part of the sign up process - such as copying and pasting it in to the sign up form.
It's hurtful to those who find their work raided and seems to give some sort of thrill to those who do it. There would be no warning - just a removal of the site and an email generated automatically informing them.