You should make use of the Thumbs Up/Down system in MailWasher in the Classify column to train Mailwasher what is good and what is spam - it's much easier. By training the spam emails as bad using Thumbs Down, and good emails with Thumbs Up you should see MailWasher correctly flagging future emails automatically as spam and good. Once that system is working correctly and flagging emails properly, you could then consider settings up Auto-delete for emails that MailWasher thinks are spam.
To do that go into Settings >> Spam Tools >> Spam Ratings >> you'll see an auto-delete option here, I would recommend first starting at around -140, which is emails MailWasher thinks are very spammy. Just be sure if you do enable auto-delete you also make good use of your Friends List for emails from people you know.