Are you sure of this? I would think that any cemetery, unless run by the city/state/federal government, would be considered the same as any other business. They can choose who is permitted entry. Just like any business can refuse service to anyone, the owners of a cemetery are able to refuse entry to anyone. Especially if they are making a nuisance of themselves.
It's my understanding that the members of this church are required to obtain a parade permit in order to mount their protests on public property. I would think that one way to keep them from disrupting funerals would be to require (for everyone, of course) one or two weeks notice before issuing such a permit. Since most funerals will not have such advance notice, the deceased will already be buried before they can legally set up their protest. And as long as everyone is given the same treatment, there can be no complaints of discrimination.
Any legal experts out there know if something like this would be possible? Would it be possible, even, to require longer waiting periods for people who don't live in a certain town, or county, or whatever boundaries you want? And what about charging fees for these permits? I'm pretty sure they already do that, but could they have larger fees for people or organizations that do not pay taxes in that community?