Okay so in order to make this run you have to first go here:
and download that.
Next pull this repo or just download the discord.ahk script.
Then double click on the script. Click on the DM or Channel you wish to clear all your messages from and then press 't'. Let it run and then if you want to stop press the 'ESC' Key.
NOTE This will not handle "Call" lines in your DM....so don't call people. Cheers!