I'm a big stickler when it comes to guests taking pictures and being in my shot. I don't care if they sit in the seat and take pictures during the ceremony. But I don't let it fly during the portraits. If I see someone getting up during the ceremony to get different angles, I will walk over to them and ask them to sit back down, and tell them that prints are available for purchase through my website. During the portraits, I'll let a few people snap a couple shots with their point and shoot cameras, but if someone is there with an SLR and they're taking shots of every pose I set up, then I ask them to stop. If they won't, then I tell the couple that their free prints are no longer available, and they will have to purchase everything they want. I warn the couple that I will say that, but it isn't true. Usually the person that is the problem will feel bad and stop, and then everything is cool. But if it doesn't then I pack up my stuff and leave.
If I had been the OP, I would have gone to those people the minute I saw them, and explained the situation. Most of the time people are pretty cool about it. If they give you attitude, then go speak with the bride or groom, and tell them that if the problem isn't solved, then you'll be leaving.
I agree with Dermit, it is a good idea to have a claus in the contract about other "photographers" and distractions. I have one in mine, and since it's been in there, I've only had one person really bother me.