That would definitely be handy. I added enough RAM that it's not really a problem now, but for people with speed problems late in the game, the leaders popping up is one of the biggest slowdown problems. And while they generally make much better offers in this edition than in CivIII, I still find that late in the game there is almost no offer they will present that I would accept.
An in-game toggle switch to turn off leader interruptions would be perfect. (If there already is one, please do not kick me in the Jimmy. Can't really check on it at work . . . )