They're moderated by whoever created them. I imagine if they permit something that is sufficiently egregious the instance creators/owners might delete the magazine? In the meantime, a user can block any particular magazine.
afa a magazine being take over. A similar process as with r/Canada and r/onguardforthee would happen: a new community could be formed. It could even the same name, just on another instance (e.g. [email protected] vs [email protected])