Permanent signature bans
For me that's the most reasonable,when a user is caught plagiarising,such user is banned permanently and isn't allowed to create any other alt account or such would be banned also(ban evasion)

Thus if we're to change things here,preventing such users from earning an extra dime on the forum is synonymous with a permanent ban,just that they will be allowed to stay on the forum,but on read mode,the only difference between it and the normal status quo is that they'll be allowed to make posts,reply posts and send PM

That way we do not change a whole lot of things all at once, and it would still act a strict deterrent as over 80% of users are all on signatures(earning from the forum),and wouldn't want to lose that privilege.