I would suggest a message asking them to edit their post, at least for the next few weeks (then you can likely do it yourself). If you start to flag a post, the interface makes it easy to send a private message to the poster.
Flagging has an option to indicate that a post is off topic. This sends a message to "staff". If if it is off topic and can be fixed by changing its category "staff" can change the category and send a note to the original poster. I'm not sure who "staff" is though...