When you try to activate an objective and someone interrupts you, just ignore them.
Keep re-activating, soon they'll loose interest and go away.
Actually I do that alot on Voidstar, keep planting the bomb regardless of getting attacked and interrupted. And it actually works sometimes.
The idea is that you keep on planting the bomb and eventually your team will kill the enemies interrupting you. Works with me alot of the time.
(No seriously I'm not joking it does actually work sometimes