Unfortunately I don't think there is a way you are going to resolve something like this. You will always have petty vindictive clients that will make nasty snide remakes. Even if they had a great time. It is just the way these bitches make themselves feel better about themselves and to justify spending the money.
But I do agree with you, that claiming someone is a meth head just because they are dissatisfied is pathetic. I had a provider in Florida once that said he was in to kissing, cuddling, sucking and fucking. But when he got there he completely changed it to only a hand job, nothing else. I gave him a negative review for it but never called him names. He said he never promised any of that and that I was a liar, until I produced our conversation, then he blocked me.
I guess what I am saying is that it goes both ways, but I can see it being predominantly more against the providers.