AntiSpam: prevent attempts to punish a user multiple times

A user may manage to send multiple message that violate filters before
the mute is applied. Because of a race condition, subsequent punish
attempts did not detect the mute role exists and therefore proceeded
to apply another mute.

To avoid the race condition, abort any subsequent punish attempts while
one is already ongoing for a given user. It could be possible to wait
instead of abort, but the first attempt failing very likely means
subsequent attempts would fail too.

Fixes #902

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