Hello Doom,
I think you want to use the Remove party member function call, this and many others are in the nwscript.nss;
RemovePartyMember(int nNPC);
Since you don't know which Party members the player will have in the party at the time so to remove all possible ones you could use something like this;
void main()
{
RemovePartyMember(0); // Bastila
RemovePartyMember(1); // Canderous
RemovePartyMember(2); // Carth
RemovePartyMember(3); // HK-47
RemovePartyMember(4); // Jolee
RemovePartyMember(5); // Juhani
RemovePartyMember(6); // Mission
RemovePartyMember(7); // T3-M4
RemovePartyMember(8); // Zaalbar
}
As far as setting the NPC's selectability you could try this function in your script;
SetNPCSelectability(int nNPC, int nSelectability);
As far as the Transit system for custom modules, this thread (
http://www.lucasforums.com/showthread.php?s=&threadid=140253) you might find useful.
I hope this helps! :D