I doubt you can block AOL system messages, as they come through the network itself and not from a "mere" user. Ignore as usual.
As for the porn solicitations, I get those too, quite often. I think it started after I entered a chat room a while back. I just choose not to accept messages from screen names with girls' names and lots of numbers.
• 2.5 GHz MacBook Pro Core 2 Duo, 4 GB RAM, 320 GB hard drive, runs 10.6.8
• 1.6 GHz iMac G5, 1.5 GB RAM, 120 GB hard drive, runs 10.4.11 (out of commission)
• iPhone 4, 16 GB, iOS 4.3.5