I have yet to see a filtering application that works 100 percent. The signal to noise ratio isn't so great on a list such as Extropians to warrant such a drastic step. I'd be much more concerned with the issues of control and manipulation of the filtering program than the real or imagined problems with off topic rantings.
If filtering is to be done, let it be individualized filtering performed on each others own machines.
For further info see The Censorware Project http://censorware.org
Regards, Matt-