It would be great if we could..

[scripaca-alpaca]

Delete specific user messages from every channel, not limited to one channel.

[ghost]

Undiscord already has the option to delete specific messages from a user.

You can delete specific user messages by specifying their ID, the guild ID, and the ID for the specific channel in that guild.

[victornpb]

I suggest clicking the (?) in front of the fields, they will forward you to the instructions page about each option