I'm really surprised they authorized wimax at 3.5Ghz, knowing the interference issue was real. (but then again, C band "satellite" is "secondary use" in a lot of countries.
If you know where the wimax tower is, you may have to put up a shield. A wall of metal screen between it and the dish. Block the wimax signal from the dish. Then, more shields may be required to block the remote bases also. I've read in other forums, that some have minimized wimax interference by using a Norsat LNB. Saying they have better "out of band" rejection. Also, if band coverage is not needed down to 3.4Ghz to use a standard 3.7-4.2 Ghz model instead of the "wideband" model. This sight
may be of some help.
Satellite Interference Reduction Group
Another site thread I've seen cover this issue is
C-Band wiped out by wireless modem - Free to Air Satellite Forums (Asia Pacific) - Sponsored by aDigitaLife.com Audio Visual Suppliers
I'm so far out in rural, it will be a cold day when wimax arrives, so don't think I'll ever see it. BITD, some terrestrial microwave was around but that "dried up" fairly quickly.