Yes, that should be rather easy. You serve a banner with your current ad serving system that will be blocked by the adblocker. The banner consists of a script that disables a function that would otherwise be executed.
The otherwise function is a simple script that redirects all current pages to the registration/login page.
From how I understand your point of view, a user with adblocker can be useful to the community if he writes content. But a guest is not giving so much to the community back.
click ratios show* that adblockers aren't a that huge problem as many administrators always say...
and don't forget that usually only power users use adblockers. that means people not clicking on ads anyway.
see, internet explorer still has a fair amount of market share and like no easy adblocker is available. and firefox/chrome do have a popular adblocker as a plugin, but it's opt-in. standard-users often don't even know those addons exist.
especially the standard user from google usually doesn't know about. i would just live with it. and what do you do if a user has no scripts at all? there are addons for this as well.
* = with adblock enabled, ads aren't even loaded. so show-to-click-ratio would increase after adblockers became popular. but over the time, show-to-click-ratios decreased, at least to my experience.
I think this is probably the worse ideal I've heard in terms of Ads and Forums. Block an entire forum if adblocker is being used? Users will just in turn ditch your site all together rather then making sure they see your ads.
Sounds to me you need to work on your site and it's ad placements and types. You need to shift your focus on your site, it's content, and like I said banner placement and types and not worry about the minority that "might" be using AdBlocker.
I was thinking, if it's possible. The Idea with the picture is a good point to start.
Disable the Forum is one of many thing, you can do, if you detected that the user has turned it off. Just answer my question, don't tell me, how bad my idea is. I want to diskuss the technical mechanics and possibilities, not the idea. Thanks.
if you want my honest opinion
I love adblock some website are full of ads and most of the time you can't navigate the content. But no matter what happen, if you have a good site with great content you will reach million of users and get support by many of them.
Sounds like a zero-sum proposition. If they use an ad blocker, they don't see your ads or click on them, if you ban them they don't see your ads or click on them. What do you gain? I think you're chasing your tail and getting rid of members who might be making a contribution that brings in others.