I definitely would be against any type of ads since preservation of site integrity should be a common theme that separates you from other such outlets. Accepting donations yearly/bi-yearly and/or getting a cheaper (or donated) host are always better options for practical results.
Additionally, going "off the grid" just to minimize user-base and conserve bandwidth is generally NOT a good thing. I say this with conviction based on personal experience from my own half-hearted game development community forums. Yes, you would definitely see a decrease in overall traffic and thus users who just leech your content, but you'd also see a decrease in the ever-so-often intelligent researchers and developers that might frequent as regulars and help expand the core concepts I'm sure you started this site for to begin with. Besides that, if your bandwidth isn't largely coming from file downloads and more-so from script requests, I hardly think THOSE users are to blame since they probably just register, visit the site once, get a file, and then are gone forever.
If you are only tipping the scales in the 80GB bandwidth range, you shouldn't have an issue with a solid shared hosting package at a dozen locations. I use dreamhost
personally and before they decided to tag themselves as "unlimited" bandwidth, my static number fell in the 2TB a month range -- of which I managed to tip a quarter of on a couple months. More importantly in the case of an active forums like this would be CPU usage (since it's a shared box). No, I cannot fully guarantee that you wouldn't use a gargantuan amount of CPU for your forums; however, I can say that I too run an old phpBB2 forum with a few thousand users and somewhat similar activity levels as this one without any hassle. It would certainly be worth a shot and a worthy investment to give them a try.
Then again, I AM one of the described people who basically was dropped a link for this site by a non-english-speaking 3D artist I hardly know in an effort to help finish up some code for conversion of an obscure file format. The only reason I'm really writing this post or even remember this forum is primarily because I found an email relating to donations being requested. While I only have $1.73 in my Paypal (which I could still donate if need be), I'm not against donating bandwidth, file hosting, or site backup/mirroring (which would probably be detrimental in your current state).
At any rate best of luck finding an overall solution.