I don't know your site and I don't know the ads running on it. I feel like the majority of the people posting here don't have a problem with ads - just with specific types of ads that have started taking over a lot of websites. Now, maybe you only have very limited, very non-intrusive ads. In which case, sorry that your ads might get blocked by people, you probably don't deserve that.
However, a lot of the ads are very much intrusive and mostly just a pain to the users. As long as a lot of companies / websites keep using those, a lot of people will keep trying to block them. And maybe you feel it's theft, but would you honestly feel better if all those people would stop visiting your website?
Ultimately, I think it's the piracy in games / movies argument; the people that are currently 'stealing' access to your website were likely never gonna use it without the adblock anyway. I know I would have stopped using youtube a long time ago due to the stupid amount of honestly very frustrating ads.
P.s.: users leaching but giving nothing in return? For a forum? How about all the free content your users have provided you with over the years which you didn't pay them for?
But if this is true, then why are (most) website not 'paid to access'? You wouldn't have to put up ads (which in turn can't be blocked so your income in unaffected), and people would pay fairly for your content. And if they don't see the value, they can simply not visit, no harm done. Seems like it would solve all the issues, so why do you think this doesn't happen?