I would see a proxy as being, from Wikipedia's point of view, like an
ISP. It would be like aol.com or, more analogously, momandpopisp.com,
some ISP with a number of users. If one misbehaves at Wikipedia they
probably don't block the whole ISP. That would be an unfriendly action
that would give them a bad reputation. Instead they probably make an
effort to contact someone at the ISP responsible for abuse and tell
them about the user who caused trouble, letting the ISP block him.
Only if an ISP were persistently unresponsive to abuse complaints
would they be justified in blocking the entire ISP, and I imagine that
this is exactly what they do.