Ask God to protect your heart from the temptation of bad or evil thoughts.
There are times in our daily activities where we encounter a person or situation that incites within us bad or evil thoughts.
These thoughts could be temptations against purity, or they could be thoughts that are directed at a particular person, desiring their harm.
Whatever category these thoughts may be in, God desires that we have good and positive thoughts that bring peace into our hearts.
Here is a prayer from the St. Andrew Daily Missal that helps combats those bad thoughts.
Almighty and most merciful Lord,
favorably consider our prayers,
and deliver our hearts from the temptation of evil thoughts:
that we may deserve to become worthy dwelling-places of the Holy Spirit.