We have a network of 100+ Q&A sites. A user active elsewhere on the network joins a new community and starts posting non-friendly answers and comments. A moderator contacts the user. The user states that the one who does not understand the rules is the moderator, because the user has behaved in this way on other sites all the time without any problems. The moderator does research and it turns out to be true that the user behaves rudely (according to the standards of this community) on other sites as well.

What should moderators do in this context?