Although it's a good idea, it will catch some innocent users out. It will probably harm the innocent more than the guilty, in fact. "Users" posting web links to Internet drug sites and adult sites are probably not users at all, they're probably web-bots. One of the best ways to prevent them is using a
CAPTCHA test. I'm not sure if there's a plug-in module for phpBB though (i.e. one that will work with this forum).
Another possibility along the lines you were thinking is to implement a policy where users must sign-in within a certain time period otherwise their account is closed. I think Hotmail opperate a policy like this. It's generally good practice to notify users of a policy like this when they initially register for a service. So they can give special notification if they're going to the Moon for a year or something similar! ;-)
For the general aspect, I'm pretty sure that it's impossible to track whether someone uses siliconhell.com, but doesn't use the forum.
Not an easy problem to solve! :-k