If you see a registered user posting some off-topic content on the forum, it is recommended to notify them on their message wall instead of doing so on the thread itself, as that your reply on the forum is moving the discussion even more off-topic.
Also, there is no need to be offensive or rude at all, no matter if the other user shows a bad behavior or not.
BTW, if two (or more) users get info a conflict and staff people or VSTF are asked to solve the problem, they can easily check the users' contributions on all wikis. They less likely support the user, who comes into conflicts across several wiki projects. I have seen you posting some rude/offensive replies on other wikis too, so I would recommend you to change your approach.