This script is discontinued and only kept for archival. Facebook's UI changes too fast and fundamentally for me to care.
This userscript replaces the archive button in Facebook's thread overview with an actual delete button. It is a fork of http://userscripts.org/scripts/show/106261
First you need a userscript engine.
- Chrome already has one included.
- Use Scriptish or Greasemonkey for Firefox
Then go to https://github.com/untitaker/facebook-delete-messages/raw/master/fbdelmsg.user.js It should be pretty self-explanatory from there.