I think both of the contradictory suggestions might work in different situations. I know when you get a song stuck in your head, professional jingle writers suggest either singing the whole song through to bring "completion" or to substitute the song with another (or something else).

Why is one talking about the ex/"Dear John" in the first place? I would work to replace thinking about him with something else and if someone else asked about him, I'd explain we had broken up and I "didn't know" and change the subject.