You can set any terms you like in your use license. Whether they are enforcable in a civil court is a separate question. For example, all through the '80s Oracle Corp. had a restriction from including Oracle product in any product comparison, such as benchmarking. Sometime in the early '90s Oracle Corp. lifted that restriction. It may have something to do with the fact that Oracle was finally running fast enough to look good next to Informix, et. al. Microsoft picked up on this mode and had similar restrictions inserted into SQL Server licenses. I didn't know about that part of Microsoft's license until I heard Larry Ellison talking on a radio interview, blasting Uncle Bill for this dispicable practise!