Sometimes, it is a conscious effort to send away evil thoughts from one's mind. That was why Jesus had to ask the Pharisees sometime during His days "why do you entertain evil thoughts I'm your mind? ".
With an innocent mind, it is sometimes naturally easy not to conceive evil thoughts, but, some other times, even with the innocent mind, such thoughts will still come unbidden.
So, it comes down to one fact here, and it is that: it is our part to send away or rather not entertain evil thoughts in our minds. The fact that this thoughts come to a person is not what makes the person bad, but, rather, when the thoughts become entertained and acted upon.
There is the saying in the Bible that: "Make a tree good, and it's fruits shall become good.". We make ourselves good by not entertaining evil thoughts in our hearts and by that, the results of the works of our hands becomes by the grace of God, good.