You could add a Jscript hack to the forums CSS file, to cap the expanded width. If I remember right it may be as easy as adding an HTML tag like DIV, as a wrapper around the entire forum code, and imbedding the CSS in that TAG manually also.
There are also legitimate CSS tags to do this, however not everyone will be using W3C compliant browsers, even in these days unfortunately.
I did this a while ago on one of my site layouts, which I allowed to expand if the screen allowed it, but would still stop at a certain number of pixels.
Yeah it's messy, but it could be a solution for huge monitors.. That way it only expands out to a certain width even on large monitors.
Alternative, on the user end you can just unmaximize your browser window and size it accordingly.
Here is a link I used for research
http://www.svendtofte.com/code/max_width_in_ie/
As far as I remember, this may work in all browsers, and not just IE.. however I could be wrong.. in any case it's just a matter of using a legit CSS tag for compliant browsers like mozilla, and using the hack for IE.
Hell, I haven't even tested the CSS tags in IE, so it may be that they work and you won't have to hack it too...