On most of the big networks there is no right to 'own' a nickname, and it's first come first serve. On some networks however you can register a nickname so that only you may use it - and if someone else does use it, you can 'ghost' them (kill them off the server). Ask in the #Help channel of the network you use to see if they support nickname registration.
If someone else is using the nickname you normally use however, you will need to just wait. You could use a script to change over automatically when the nick goes offline, see /help on unotify.
Regards,