You can do this from a story script. Something like:

Code
IF
GameStarted(_,_)
AND
DB_IsPlayer(_Character)
AND
CharacterIsControlled(_Character, 1)
THEN
CharacterRemoveFromParty(_Character);

So, when the save loads, whatever character you were last controlling when you saved would be removed from the party.

There's a safer way, but for a quick fix, eh. This just requires some minor prep.