First of all, stop putting things into your imagination which are evil. Remember, things are either good or bad, there is no neutral in the spiritual life.
Second, use custody of the eyes and ears.
Third, control curiosity and see my posts on this and some reposts.
Fourth, ask God to enter into your memory to purify it and take away images. Images are used by demons to tempt us and remind us of past sins.
Today's Gospel refers to not even committing venial sins, which also feed our imagination as well.
17 Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.
18 For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
19 Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.