I have the script:

void main()
{
object oNPC=GetObjectByTag("p_Kicht");
ActionDoCommand(SetCommandable(TRUE,oNPC));
AssignCommand (oNPC,ActionDoCommand(DestroyObject(oNPC)));
}

running when my conversation ends. I also had,

void main()
{
object oNPC=GetObjectByTag("p_Kicht");
ActionDoCommand(SetCommandable(TRUE,oNPC));
AssignCommand (oNPC,ActionDoCommand(DestroyObject(oNPC)));

RemoveAvailableNPC(1);
AddAvailableNPCByTemplate(7, "p_Kicht");
DelayCommand(1.5,ShowPartySelectionGUI());
}

this running before but the only thing that happened was my npc joined me but the original one didn't disappear. And now that I only have the destroy script running nothing happens. Could someone tell me what I am doing wrong?

I'm relatively new to this whole thing, but I think the problem is that you have the tag as "p_kicht". You're thinking of the ResRef. Look at the .utc file again and look who you have as the tag in the field under "last name".

Having spent the last 2 days trying to fix this same problem with my mod, I just now got it to work by fixing this little oversight.