How can you politely and respectfully interrupt someone who tends to talk excessively while in a bar setting?

You just have to cut them off. When they put a period on one of the sentences you jump in front of their next sentence and you start talking.

It’s likely a loud/chaotic bar setting, so don’t be afraid to turn up your volume a bit and talk louder than them. If they increase volume, you do so again. Stay above them, cut them off, and don’t react when they try to recapture the convo, just keep on saying what you gotta say.

It will be awkward at first, but eventually they’ll get the message. Or they wont and you’ll both just be yelling at eachother in disconnected simultaneous convos. If it’s that bad and they really can’t read the social situation, just politely leave and go talk to someone else who isn’t hogging the conversation.