Keelboat sailors on a race think of the windsurfers as 'sailing into their fleet', causing hazards; ie they think its our responsibility to keep clear of them. If the course is marked the competitors do not feel responsibility to keep off 'their own' marked race course!
The only and obvious solution is to have a good chat with the committee of that club. Get a copy of the same chart they use, draft on it the area the windsurfers use in each wind direction, and take it along. You may find they have a course charted for each wind direction, and negotiation may result in it being tweaked.
Of course, if you just take a gang of windsurfers through at 30 kts screaming 'STARBOARD', 'WEATHER BOAT', 'WATER ON THE MARK' and the like, relations may deteriorate.