Using ChanServ's MLOCK:
<@FiXato-AFK> ? mlock * [WatchdoG:#help] mlock == Sets the mode-lock parameter for the channel. ChanServ allows you to define certain channel modes to be always on, off or free to be either on or off.
/msg ChanServ SET #channel MLOCK modes
Where modes have to be replaced by the modes you want to be either locked on or of, for example +ntS-lips
/msg ChanServ SET #channel MLOCK +ntsk myKey
to lock the modes n, t, s and k with key myKey.
(n = no external messages, t = only operators may change topics, s = secret, k = key) to remove the mlocks, just do the command again without parameters or limited parameters, e.g.:
/msg ChanServ SET #channel MLOCK +
to unset all MLOCK modes
/msg ChanServ SET #channel MLOCK +nt
to set the default MLOCK modes.
You can also force modes not to be allowed on your channel by using a - sign instead of a + sign:
/msg ChanServ SET #channel MLOCK -s
won't allow the channel to be set to secret mode.