Were this happening on one of the Wikis I manage, I'd approach it along the lines of...
- Thought I'd send you a non-automated welcome to this wiki and to thank you for joining us. I noticed that you've been adding and then removing Categories to pages, can I ask why you're doing this.
- We've had people in the past just to gain badges, rather than because it actually helps contribute to the Wiki, but I wanted to give you the benefit of the doubt and ask you if there was a reason that you were doing this (I like to assume that people are trying to help).
- If you are keen to earn badges, there is HEAPS to do. We have a To Do List that has a few suggestions of things people can do to help make this place better.
- Thanks again for joining us. I look forward to seeing more of your contributions about the place.
- Raz : )
But that's me. I have a very informal approach, which doesn't mean that I can't call people on their BS when necessary, but I have encountered instances where trying to assume the best of people has actually paid off, and that what someone was doing wasn't what people thought they were doing.
Hope this helps. Feel free to plagerise and re-word my suggestion to suit your own culture and mannerisms.
Raz : )