Pruning is a more involved method of deleting messages that can only be done by the channel owner. It allows you to delete system messages as well, which include those that describe any changes made to a channel's structure.
To prune messages from a specific user or between specific time frames in your channel:
Click on the three vertical dots in the top right corner of your channel & select "Prune Messages".
Set your parameters for the pruning (be sure to double-check the dates & times, or username).
Select "Prune" and confirm to prune messages.
Pruning messages is permanent and cannot be reversed, so we recommend being extremely careful with it.