I had a similar problem at my office on a couple occasions. Certain people would just butt in to whatever conversation they felt like. No particular reason other than they heard some people talking about something and felt as if they could add to the conversation when really they were just being annoying.

The solution we used was to point out that they had jumped in to the conversation every time they did it.

Person 1: Blah blah blah blah
Person 2: Oh really? That's cool.
Interrupter: Well I heard that blah blah blah was actually yak yak yak.
Person 1: Jump-in # 1 for today!

After a while the Interrupter got the message. h: