When a party member is locked, you cannot remove them from the party using the trading scene.
However, you can still forcefully add or remove them from the party using event commands or script calls, because those are things players should not be able to do.
With the Party Manager, you can create and manage multiple parties.
By locking actors to each party, you can create a situation where the team must separate into different parties, and each party must have certain leaders. The player can then proceed to choose who they want in each party.
Need some party management functionality? Try these plugins out!
Party Manager: http://himeworks.com/2016/02/party-manager-mv/
Party Trading Scene: http://himeworks.com/2016/03/party-switching-scene/