I think this brings up a good point... you seem to be interested in filtering outgoing web requests, so that your spouse doesn't have to look at the nasty pics outlook insists on showing her. But wouldn't it be better if she never had to look at the email at all?

The solution I use to get rid of spam is SpamAssassin which while it has it's faults, but it has worked well for me after some tweaking. The popfile program that meetraz mentioned looks like it would be closer to what you really need in this situation. If it had SpamAssassin in the middle of it instead of it's training method (or had both) I'd call it inspired. (IMHO, it would be better if it acted as an IMAP server, so that you can have your SPAM in a different box right off the bat, but whatever.)

In any case, a simple filtering proxy server might have other handy applications, but the maintenance overhead seems like it might make it a pain. (Adding the offending IP addresses, which quite likely change often.) With SpamAssassin, I haven't had to tweak anything in months. However, with solutions like popfile and SpamAssassin you need to check what it's marking as SPAM periodically to insure that hasn't canned something you really needed to see.

Good luck...