1. Sign in to Gmail.com
2. Open the settings by clicking the settings icon at the top right side. (new version)
3. Click on Filters tab and then click on create new Filter.
4. A form will appear to create a new Filter. Leave everything blank, just write in:spam in the Has the word part. Click on Create filter with this search .
5. In next part, click on the checkbox
"Delete it"
Also click on the other checkbox if you want to delete already e-mails in spam folder.
And click on button Create Filter