By clicking on the "Confiugure SpamAssassin (required to rewrite subjects)" button, you will be taken to the advanced configuration menu.  The options here can get very detailed, but for most people, simply changing the "required_score" will be all that is desirable to touch.

required_score  les you adjust the filter's sensitivity.  A lower number means you get less spam.  A lower number may also mean that more legitimate email is filtered inadvertently.  We recommend a standard setting of "5" but we've had clients run as low as "-1" before.

If you have any questions please let us know!