From WoWWiki

The primary command to remove a member from a party (or group) used by the party leader. The target of the uninvite cannot block the removal.

Ways to uninvite:

• Select a party member character, right-click their portrait and select Uninvite.
• Typing /kick [player name]

If the leader uninvites the only other character in the party, that effectively disbands the party.

Also see Invite and Leave Party.