CP -> Display Options -> Styles -> Theme to Edit -> Style Settings -> UBBCode Properties -> .ubbcode-body -> Change max-width and max-height to both be 100% and remove the "overflow" property.
I wouldn't recommend this (at least disabling overflow) because it'll allow your forums to "streach" to accomodate content (such as large pictures).
Well, before we upgraded we had an OLD version with no scrollbars ... and on the negative side, large pictures now and then. It seems that our users rather would live with the large pictures if they can get rid of those bars.
I'll use that option for now, and hope someone will come up with a more flexible (or, ehe, less flexible, as it were) suggestion.
Ahh, you're running the "new style" ubbcodes, I use the "original style"; they where adjusted in 7.1 due to an IE compatability issue (I don't even remmeber why exactly); try changing your .ubbcode-body's max width to 100% to see how i was planning for it to look
The IE fix was to explicitly specify a width, I hate it, hate hate hate... it makes multi level quotes streach pages versus incorprate into themselves...