We call that an ear-worm. I have two theories.

1. Try to banish it by having other music in your life. If you play or sing, play or sing something else. Or put other music on the radio. Perhaps that will displace it.

2. Take an allergy pill, such as Claretin. When allergies are bad, the cerebral-spinal fluid is under excess pressure. I have a theory that the pressure is irritating the brain cells that store the song in the ear-worm. (When a body part is irritated, it responds by over-doing its job. )

This is my own theory, and it has no basis in medical research or doctor's advice. But doctors probably don't regard ear-worms as important, at least not as important as heart disease, say, or breast cancer.   

3. Other people think that if you simple capitulate and sing the song, that it will go away. Not sure if I believe that myself.