I assume you are asking how the Thought Police prevent rebellion. The why doesn't really have examples -- it's just because that's what the Party wants.
As for how, the Thought Police does this by arresting and retraining or just killing anyone who either does something or seems like they might do something harmful to the Party.
For one example, Winston is arrested and retrained because he has been having an affair with Julia. The Party sees this as rebellion, so the Thought Police catch him.
A second example is Syme. So far as we know, he does not do anything. But he is just too smart and thinks too much so he is a danger. In this way, the Thought Police head off trouble before it starts.