Oh my, I'm prone to the same thing, though the song tends to change. Very, very clear and distinct in my mind. This is a form of an occupying thought. These can be worry about the next day, noises in your bedroom, or anything else that detracts from basically "thinking about nothing" or just letting your mind wander without direction, which is more conducive to falling asleep.
My strategy: immediately ignore it. Don't allow the song to take root or it becomes harder to turn off. If it turns back on, turn it off. Be persistent and determined. You can also try various breathing and muscle relaxation exercises in bed, which will distract you from the song.
Even the age-old method of counting sheep will do the same thing.