Oz Rexroth

I let someone come to my place the other night and she asked if she could come back to visit. I said sure, and then she wanted to bring a friend. I have tried everything I know to uncheck the "allow others to create landmarks here" box, but it won't un-check. I don't want to come on some night and see strangers all over the place. What should I do? Is this just some sort of new SL glitch? I've never had problems with this before.