You can clean out your PM inbox and save your precious PM's at the same time. There is a feature that will allow you to email those precious PM's to the listed address under your "Edit Profile" page.

Just select the PM's you want to save or select all and at the bottom of the "Messages" page there is a button called "Email checked" once you have confirmed that you have received your PM's you can safely delete them from your inbox. Now other users may contact you privately once more.

easy peasy japanesey