The life hack I have come up for people like this is to stand too closely to them when they talk. Have a great big smile on your face, like a crazy one. If they move away, move in closer. If they ask "what are you doing" say, in an almost gay way, "I'm REALLY into what you have to say and want to soak up every word". Slowly move your hand towards theirs until it is almost touching, you don't have to touch, they'll already feel grossed out and get really uncomfortable, make some excuse and get away from you.
The next time you see them, call their name and run up to them with a big smile and get too close again. They'll get away ASAP.
I have never had to do that more than twice to get to the point where they are basically running when they see me, or jumping into a conversation with anyone else to get away.
Only tell people you really trust what you have done. You don't want it getting back to the blabber mouth, or else they might start talking to you again.
Good advice. :)