Do it by hand, or just PM your self those you want to keep. (forcing them into your out box, which is probably also full.) I myself have 209 messages, and I just ignore most of them it can't be helped for me.
"The person who is really in revolt is the optimist, who generally lives and dies in a desperate and suicidal effort to persuade all the other people how good they are." -G.K. Chesterton
Bandwagon Leader of the Hinjo Fanclub
Mine have accrued mainly because of my online game. I advertise here about playing and people send me PMs asking about the rules, or about their character concept, etc. They have my email address, but for about two weeks that wasn't working and I think some of them just got in the habit of PMing me.
Most of the letters are inconsequential. I started deleting them the other day, but I was on dial-up and it seemed excrutiatingly tedious. I was hoping there was a better way.
New Terminator movie = Awesome!
There are two choices. One is to do each message manually. The other is to use the "Empty Inbox" and "Empty Outbox" links at the top of those respective pages. It's either "All at once" or "one at a time". No middle ground, sowwy.