Exclusive sponsorship's are not new or uncommon. But it should only relate to sponsorship, not other commercial dealings.
If Air Venture are refusing to deal with AvPlan commercially (ie not allowing AvPlan to take a display booth) at the direction of a third party (ie Ozrunways), then this sounds very much like third line forcing to me which is against the trade practices act and actionable in the Federal Court.
Frankly, I think its a naive and clumsy move by both AirVenture and OzRunways in such a small community as aviation. Its guaranteed to disenfranchise someone.
Ozrunways would be justified in wanting a marketing advantage from being the major sponsor (and if their assertion is correct only sponsor), but there are many smarter ways to do it.
I count 14 sponsors on previous AusFly posters. Are OzRunways really the only ones prepared to return as sponsors?