That's why I had Minsc but not Dynaheir. This trick works for any of the "paired" NPC's, like Dynaheir/Minsc, Jaheira/Khalid, Xzar/Monty, Eldoth/Skie:
1. Send the "reject" member of the pair into a room where you never intend to go back in again (where the cursor actually changes to a 'open door' icon), whilst leaving everyone else outside.
2. Double click on your PC's character portrait to shift back outside.
3. Reform the party, removing the undesired member. Since they won't have an opportunity to actually speak with you, they'll never call back their buddy either! :D
Luck be a Lady.