Originally Posted by
zlefin
Do you have those NPCs thoroughly statted up?
What is the reason you don't want the teamup to occur?
My first inclination is to simply the party, OOC, reasons why you don't want the teamup to occur.
A hypothetical example:
"I know it makes sense to teamup from a verisimilitude perspective; but DM'ing is a lot of work, a teamup requires me to alter encounter balance significantly, it also means I have to run those NPCs all the time in addition to everything else I'm doing. So let's just not do it except for very special occasions, ok?"