This command forces a user to part whatever channel they are in with the reason you give. This command will rarely if ever be issued, as channel operators should be the ones responsible for this and individual users should be free to join and part channels as they please, subject to the appropriate channel modes. This command takes 3 arguments, the channel for the user to part, the name of the parting user, and the part message that will show to other users. To force nick1 to part #channel for the reason "Be back later", issue this command:
/fpart #channel nick1 Be back later
Note: You will likely need to use quote or raw to issue this IRC command. If so, you will need to prepend a : symbol to your part reason. If you fail to do this, the only the first word of your part reason will be shown to users. If your part reason is exactly 1 word, then this is not required.