By using GMS Anti-Spam you should be able to eradicate the presence of Junk Mail from your system almost completely. Current users are seeing a reduction of over 98% in junk mail entering their system and a complete eradication of relay. Even in its basic form it will certainly ensure that no-one can 'steal' your system as a relay for their junk mail. Hence, you can reserve all of your resources and bandwidth for your own use.