Changing channel modes is an operation restricted to channel operators, who can be recognized by the @-prefixed to their name. They mostly regulate access and moderation.
Channel modes can be permanently set for registered channels through ChanServ:
/msg ChanServ set mlock <#channel> <+-cmode>
Available cmodes can be queried via:
/quote HELP CMODE
MODE <channel> <+|-><modes> [parameters] ? designates that the cmode is provided by an extension and may not be present on this server. CHANNELMODE - DESCRIPTION ------------------------------------------------------------------------ NO PARAMETERS: +n - No external messages. Only channel members may talk in the channel. +t - Ops Topic. Only opped (+o) users may set the topic. +s - Secret. Channel will not be shown in /whois and /list etc. +p - Private. Disables /knock to the channel. +m - Moderated. Only opped/voiced users may talk in channel. +i - Invite only. Users need to be invited or match a +I to join the channel. +r - Registered users only. Only users identified to services may join. +c - No color. All markup (color, bold, underline, etc.) in messages is stripped. +g - Free invite. Everyone may invite users. Significantly weakens +i control. +z - Op moderated. Messages blocked by +m, +b and +q are instead sent to ops. +L - Large ban list. Increase maximum number of +beIq entries. Only settable by opers. +P - Permanent. Channel does not disappear when empty. Only settable by opers. +F - Free target. Anyone may set forwards to this (otherwise ops are necessary). +Q - Disable forward. Users cannot be forwarded to the channel (however, new forwards can still be set subject to +F). +C - Disable CTCP. All CTCP messages to the channel, except ACTION, are disallowed. ? +O - IRC Operator only channel. ? +A - IRC server administrator only channel. ? +T - No NOTICEs allowed in the channel. ? +S - Only users connected via SSL/TLS may join the channel while this mode is set. Users already in the channel are not affected. WITH PARAMETERS: +f - Forward. Forwards users who cannot join because of +i, +j, +l or +r. PARAMS: /mode #channel +f #channel2 +j - Join throttle. Limits number of joins to the channel per time. PARAMS: /mode #channel +j count:time +k - Key. Requires users to issue /join #channel KEY to join. PARAMS: /mode #channel +k key +l - Limit. Impose a maximum number of LIMIT people in the channel. PARAMS: /mode #channel +l limit +v - Voice. Allows a user to talk in a +m channel. Noted by +nick. PARAMS: /mode #channel +v nick +o - Op. Allows a user full control over the channel. PARAMS: /mode #channel +o nick +b - Ban. Prevents a user from entering the channel, and from sending or changing nick if they are on it, based on a nick!ident@host match. PARAMS: /mode #channel +b nick!user@host +q - Quiet. Prevents a user from sending to the channel or changing nick, based on a nick!ident@host match. PARAMS: /mode #channel +q nick!user@host +e - Exempt. Allows a user to join a channel and send to it even if they are banned (+b) or quieted (+q), based on a nick!ident@host match. PARAMS: /mode #channel +e nick!user@host +I - Invite Exempt. Allows a user to join a +i channel without an invite, based on a nick!user@host match. PARAMS: /mode #channel +I nick!user@host