When that person tries to hijack your work, I think you could choose to stay standing (presentation, right?) and say something smooth like, That’s ok, Ill take it from here—insert name here. Then start right in saying its your work like you both rehearsed it that way. The smoother you can make it, the better.
In the personal situations, I would say put limits on the people you dont want to drop, and let the others play out their little dramas on their own. I am trying so hard to do this myself!