CHANNEL MODE d

Usage: MODE <channel> +|-d
d - Block nick changes
This mode will only allow 1 nickname change per user, from Guestxxxx to a nickname. It does not affect network operators. This will keep a channel from getting cluttered with nick changes. It is particularly useful in larger channels. You will probably want to set channels +d if you are having a moderated event.
Privilege: channel operator
Examples:
/mode #movies +d