Excellent advice from Rose Eveleth on dealing with someone who won't let you talk:
"There are also more advanced moves. One of my favorites is the Question Sneak Attack. While your monologuer is talking, say over them “Jim (or whatever their name is), can I ask you something?” This often makes them stop, or at least wrap up their thought. Because there’s nothing better for an over-talker than you asking them a question. This makes it seem like they are not simply holding forth at length, but instead answering your questions. When they do stop, you don’t, in fact, ask a question. Instead, you make your point. Every time someone successfully executes off the Question Sneak Attack on the radio I high five myself, or whoever is around me. It is probably my favorite move.
A less advanced, but very effective move is the Name Drop. When you find yourself unable to get a word in edgewise simply start saying the speakers name. Like this, “Jim, Jim, Jim, Jim.” This is incredibly effective because it is usually a short and easy set of syllables to get in that makes it very clear that you’re being talked over. And because you’re using the talker’s name it is very clear that you are addressing them specifically. Eventually, they will have to stop and say “yes?” At which point you can politely point out that they have been speaking over you this whole time and you did not buy tickets to this one man show and could he please show you the exit."