How about letting all of your users know via mass email that everybody's inboxes will be cleared out, and to export their emails as a CSV file if anything in there is important? And backup the board just in case?
Personally I don't see any reason why that wouldn't work.
Yeah, but how? There's no command to do that. True, I could go into the database and try emptying all the fields that are related to PMs, however, even reading this topic you'll see that there isn't total agreement on which database fields are involved or what the effect would be.
I'd like to see someone fix the code shown above. This would be a perfect solution if it worked.