How should I handle it if an artist goes badly off-specification with a piece of artwork?
Even if they are given corrections, and manage to go completely off-spec on those too?
This seems like it's time to reject it outright. Seems like if an artist submits art that doesn't meet the specifications, then you shouldn't pay for it because they haven't given you what you asked for.
(I'm aware that there can be communication problems that cause this and that should be investigated too, specifications might potentially be unclear or misleading, but I'm specifically talking about when specs were clear)