/mode # +t will only allow operators to change the topic. Locking the topic so that it cannot be changed is usually set using the channel services (ChanServ, ChanOp). /msg ChanServ HELP SET TOPICLOCK
/msg chanserv help set topiclock Usage SET #channel TOPICLOCK [off | founder | aop | sop ]
/msg chanserv set #channel topiclock aop /msg chanserv set #channel topiclock sop /msg chanserv set #channel topiclock founder /msg chanserv set #channel topiclock off