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
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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