Vote manipulation is getting more common. Some recent examples:
While the accounts were banned, the malicious voting activity stuck around.
Should admins have the ability to discard votes, and if so, which admins? Should community mods have that ability? Can you think of any ways that tools like this could be abused?
It depends on the reason for banning, no? If the account was banned because it is a bot, it makes sense to remove all their activity including votes.
However, if the account was banned for misbehaviour, I think it makes more sense to remove only the offending posts and directly associated votes. E.g. all votes by the offending account in the thread in which the offence took place