If you have an e-mail account on the imap server you can now have messages automatically deleted from your Trash folder as well as from your Spam and Virus folders.
Visit the spam/virus/trash e-mail deletion
Don’t currently have your spam and virus e-mail automatically filtered by the imap server? Head on over to the automatic e-mail filtering
If you ever need to get to them again, both of the aforementioned utilities can be found on the forms page