Please allow us to ban members who have left

Every once in a while some weirdos raid our server, but we don't have time to do anything about it because they leave before we can ban them. It might be possible on desktop, but it certainly isn't on mobile. That creates a big problem, because when people start spamming stuff in the text channels we also have to manually delete everything, instead of just banning and removing last 24 hours. There should also be an option to remove all messages sent by a certain user within a specified timeframe without having to ban the user.