how to mass delete messages in imessage?

To access your user Library so you can try dialabrain's suggestion, go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library.

Yes! Hold down OPTION +COMMAND, then rapidly hit delete. The messages will delete very fast.

You can't.

Perhaps this no longer works but you used to be able to go to ~/Library/Messages and delete chat.db, chat.db-shm and chat.db-wal which would delete all messages.

Thanks Eric. I should have mentioned that but I was feeling lazy.

You are welcome. 😁

Than youj so much. saved me tons of time. The people at the Apple Store didn't even know this one.

Happy 2018