On what basis?
The aggrieved party here is Null and KF. Not the pedo. If the guy is a convicted pedo, he's a convicted pedo. Not much you can do about that. Truth is an absolute defense against defamation.
Null took a ding here with that faggot having an account here and overstating his involvement and association with the site. He's not a "volunteer." He's a random ass user who seems to have mostly flown under the radar. Jannies like
@AltisticRight are volunteers. They are expected to provide a service and, as the meme goes, "they do it for free."
I'm tempted to say Null should initiate legal action against the pedo for this crap, but I dunno if there's a basis for it. He needs to fuck off forever though.