there are several ways depending on the network and if it has chanserv (channel services)
the most common way is by doing: /mode #channel -o <insert_nick>
that should remove thier ops status unless there is somekinda of bot in the channel from the network. i know the network I go to, has an additional chan/user mode that protects certain people like the channel owner and sops. try the way i showed ya and you should be just fine.
/chanserv vop #channel add <nick> will remove the person from aop and re-add them to the autovoice list if that helps. Different networks might have slightly different commands though, so it might be worth asking in one of the help channels.