There are two options that I see:
1. members pay membership fees to come to the site, and then there won't be a need for sponsors. How many of you are willing to pay that??
2. leave if you don't like it. Noone is putting a gun to your head to be here.
Some other options to consider Big A (seriously - not trying to be a dick at all):
3. Recognize that perhaps the complaints might indicate that you're doing the top sponsor a disservice if the banners are indeed annoying.
4. Consider another means to highlight the primary / top end sponsors. I don't know the details of vBulletin (or the version you're using here), but some sites will:
-Include ad's placed between posts w/in threads. These always jump out a me. Higher paying sponsors could get this or at least have their banner included more frequently.
-Banner SIZE (rather than number) can have the effect of prioritizing a particular sponsor (e.g., a banner that spans the page is another way to make that sponsor more prominent compared to the smaller 3 or 4 to a page banners).
-Including a SUB-Forum within the Sponsor's forum for higher paying sponsor's would give them priority.
-Cross-Stickying Sponsor Sub-forums to other forums would give them more visibility.
-Then there are more effortful ways to advertise like holding contests on the site with sponsors giving away prizes, having featured sponsors (like featured members) at the top of the page, and things similar to what you do with the PM's r.e. synthetek products (and other stuff that could be annoying to some people, like pop-ups, etc.)
Personally, I'd rather get a PM a week about a particular sponsor a/o its products vs. seeing 10 identical banners and would be more likely to use that sponsor, I think. With the PM's, the exposure is different / variable (not the same set of banners each time), yet intermittent but frequently spaced enough to create a good reminder / impression and not annoy...
-Scott
P.S. Hopefully the length / effort of this post tells you I'm truly trying to help and not just be critical.