This general issue isn't specifically related to Skeptics.SE, so I went and did some spelunking on Meta.StackExchange to see how this had been handled in the past.
The general advice: Flag one of their posts, and ask for moderator intervention.
If they don't have a post, flag anything. (If you are about to comment that this isn't a pretty solution, I agree!)
There's a "semi-official" guideline to how much leeway is allowed in posts, usernames and profiles. Fake names are also discussed.
(As an Australian mod, I would point out there are large cultural differences between offensive terms. I'm not looking forward to being the arbiter on borderline cases. If it is blatantly obviously offensive, no problems, but if someone is sneaking in some dog-whistling or local term that we might not immediately recognise as offensive, help us out by explaining why you find it offensive.)