A prevert is a person who pre-empts a pervert by doing to the pervert what they would have done to them, e.g. when a ballerina takes an illicit sniff of a foot fetishists loafer or when a child rapes a paedophile
"Mrs. Robinson? Mrs. Robinson? Hi, I'm sorry to have to tell you this but your son Billy molested me last night. Molested me badly."
"Oh my god! Billy did?"
"Yes I'm afraid so."
"Hey, wait a minute, aren't you that paedophile that just moved into the area?"
"Err...yeah...Frank's the name. But I didn't initiate anything, I swear to god."
"Oh that's just mummy's little prevert taking matters into his own hands. I mean, you would have done it to him wouldn't you? Wouldn't you pervert?"
"Yeah...yeah I suppose I would."
"You have to wake up pretty early in the morning to surprise my Billy!"