How to stop words and phrases continuisly coming in my mind

What words? What phrases? Why must they be stopped? When do they come? How old are you and what kind of environment and background do you have? Do you have a doctor, or ever been treated for any ailments, physical, or mental?

Are you stressed or under any pressure? Are you depressed? Please give a lot more information if you can, as generally focusing on important things that need your attention leaves little time for unimportant thoughts. So another question is why are these words and phrases so important to you? Put another way, why do you make them important?

EVERYONE in the world has this happen to some degree. It's normal unless it's extreme, and ruins your life.
"Middle" extreme might be because you are repressing anger, hurt, abuse, a forbidden desire, or guilt.
Worst case is a mental condition such as schizophrenia.
As talaniman says, many questions need to be answered before we can help.