Log in and in the search are type something like this :
Then "Select all conversations that match this search"
Purge Verizon Inbox
Log in and go to settings >> posts per page >> no limit.
Go back to Inbox and select all messages and delete!
Purge Yahoo Inbox
Purge AOL Inbox
Comcast isn't too bad, just shift+click in the "select all" box then delete.
Purge Hotmail/Microsoft/Live Inbox
The easiest supprisingly! Just click the select all box, then click the words "Select everything" in your inbox. Then Delete!
Go to Manage Folders, then you can delete entire folders.