That's a form of Obsessive Compulsive Disorder, specifically Intrusive Thoughts. Everyone has intrusive thoughts once in a while, but if they become too often and too disturbing, like yours, they can be a serious problem.
Here's for more info Intrusive thoughts - Wikipedia, the free encyclopedia

There are some fancy psychological experiments you can do on yourself in an attempt to decrease the thoughts frequency, like purposefully exposing yourself to them until you become desensitized, do some CBT or medication, but honestly, you can get rid of them by just not giving them attention. Don't entertain them, that's what gives them power over you, the fact that you give them attention and become so disturbed by them. Don't suppress them, however. When they come, you become aware of them for a moment then discard them and continue whatever you were doing like nothing happened, without reacting in any way.