Use the check-box at the top of the list to select all for that page. Then you can delete all selected.
Go here: http://forums.digitalpoint.com/private.php?folderid=0&pp=100&sort=date&page=1 Select all for that page, repeat for the next two pages too (per page increased from 50 to 100) then delete selected. 300 will be deleted all at once.
hello, by clicking "empty folder" will remove all the PMs you have --& you don't even have to check all the messages on a single page. Isn't that the case? i have used it many times