You ought to put in a 3rd party anti-spam/anti-virus solution. Yeah you could try to continue messing with rules, but you'll find that it's a never ending battle.
Some options I have dealt with are:
GWGuardian by Messaging Architects (sits on another server and mail passes through it - highly configurable to do various levels of spam detection). Powerful solution but currently requires a separate Windows server to run on. A Linux version will be available by March.
GWAVA by Gwava (Runs on top of NetWare server integrating into MTA). Some people prefer this, I don't care for it. But it is pretty easy to configure and manage.
Astaro - perimiter solution that provides firewall, intrusion detection, etc... but also has a decent spam filter component. Not as powerful for anti-spam as GWGuardian, but for an all-in-one security solution it's pretty good. It comes in appliance form or you can install it on your own hardware. Very easy to configure and use.
Security Manager by Novell - basically same as Astaro but OEM'd and packaged by Novell. Note that it is one version behind the full Astaro product.
Another thing that would help is just having GroupWise 6.5 or GroupWise 7.0. They have a few more features not available in GW6.0 like RBL's. Just setting up RBLs can cut your spam down by 75% (rough guess)
Marvin Huffaker, MCNE