I'm not sure as to why they're doing it, but if it's something that bothers you you have every right to tell them to knock it off. You can also untag people from your pictures, and I believe they can't retag themselves if you've removed a previous tag.
Well, I'll admit that I don't get it at all. I always thought tagging was a way of "claiming" a pic as though you are the person in it. So it just seems like it would be misleading to let someone else tag my pics. I think I'll just leave it untagged and continue to remove new ones.
Thanks for the reply and info Sam!