The Four Right Exertions are part of Right Effort in the Noble Eightfold Path and are:

1. When a monk desire and exerts to keep unskillful qualities from arising
2. To abandon unskillful qualities when they arise
3. To help skillful qualities arise that have not arisen
4. To maintain skillful qualities that have arisen.

How to abandon unwholesome thoughts after they have arisen:

1. Simply abandon the thought (be mindful)
2. Remind yourself as to where these unwholesome thoughts will lead
3. Distract yourself or divert your awareness away from the unwholesome object
4. Investigate the unwholesome though
5. As a last resort, suppress the thought