How do you prevent dish antennas from being installed on condominium balconies?

Unless your HOA has specific rules about this, you may not have any recourse in the matter. It then becomes a one on one matter between you and the neighbor. Guaranteed, the neighbor will not back down about where his/her dish is sitting, so your next action would involve a lawyer. Begin things by checking your HOA architectural guidelines. The issue may be addressed there, or you may be able to build a case based on what is there.