In my experience the most ineffective thing I can do to stop unwanted thoughts is to try to stop the unwanted thoughts. Yes, I realize that's counterintuitive. The more I try not to think about something, the more I think about it.

You may be able to distract yourself, or you might examine the unwanted thoughts carefully and work from them toward other channels.

The "Mindfulness" route works for an unknown (?) percentage of those who invest effort in it.