A follow on from Problematic Players help (1/3); Newbie Declaring Solo . In that question, I covered one of the problematic players in my online group. But she's not the only problematic player, not by a long shot.
I should mention, because it's more relevant here: this group is scattered all over the world. We communicate using discord for OOC, and roll20 for IC. Everything is text.
The other problem player is the most (and only) experienced player in the group by several years... but they are also a sneaky powergamer, and often try to sneak around using whatever version of the rules suits whatever shenanigans they're up to. An example of this is rolling up their character when everyone else is made with point-buy, and then somehow (using a personal dice roller, no screenshots or even the slightest fanfare, of course) getting such good rolls that none of their stats started below 16 in the game. This was pretty quickly corrected when it was spotted, but whenever I try to confront them on something, they have a tendency to vanish into thin air until the subject changes. This makes it incredibly slow and painful to get anything from them.
What can I do to change this behaviour, or best confront it?