I would have to agree about turning the sitter in. That kind of behavior will not change till they know that punitive action will be taken against them for violating the trust.
As long as TG follows up and delivers punishment to a rogue sitter then in time it will become known that if you sit and trash someones account, you as the sitter will be held accountable.
This can't happen unless you turn them in. There is no dishonor in doing it that way. The only one dishonorable is that sitter and only you have the power to change how players like that operate.