Riot has a cool new option for room admins called "Remove recent messages" on user profiles to remove messages from those pesky spambots.
Usually, kicking a user gives you the option to remove their recent messages too. But in case of IRC spambots bridged to Matrix channels, kicking or banning those temporary users is pointless. This feature is useful in such cases.
I just hope that they bring a decent server-admin interface to the synapse server :)
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