Allow Channel Admins to be able to enter "select" or "edit" mode where checkboxes appear next to the messages.
Admins would be able to select messages one by one or to select a range of messages by clicking the first one, holding Shift button and selecting the last one (like we've used from other software).

After selecting multiple messages, Admins would be able to delete them at once, or perform any other action that might be implemented during the development (move, archive, flag, something else).

This feature would make it a lot easier to delete whole bunches of off-topic messages and make all the channels cleaner and easier to use - both from user and admin point of view.

Hopefully it was helpful :)
Best