I think if you are to decline an interview, you should probably do it as soon as possible. I would either call or respond via email and politely say that you appreciate the program's interest in you, but will not be able to attend the interview, although you thank them for their time and consideration.
I am sure these program coordinators understand that students will not be able to attend all interview offers they receive for one reason or another. But as long as you are polite and respectful (as academia really is a small world, you don't want to step on anyone's toes) I don't see anything wrong with declining an offer or two. But, out of respect for the program that was considering you, I think it would be best to let them know as soon as possible if you really don't feel like you would want to attend the program anyways.
But congratulations on having so many interviews to choose from!