When we make our bodies the dwelling place of the Holy spirit we make it impossible for evil spirits to have access into us. People who refuse to accept Christ into their lives make themselves vulnerable to the attacks of the evil ones. We should be mindful of what we let take control over us.
The actions we take are either influenced by the right spirit or a negative one. There are certain things or certain ways we see people behave that just makes us wonder how someone could behave in such manner. In most cases it's not the person that chose to behave that way, it could be that the person was under the influence of a negative spirit that prompted him or her to act that way.
The way we live our lives matter a lot. There's a way a believer would live his life that would keep evil spirits from gaining access. But there are those that live life anyhow and the enemy will have his will over them without even seeking for their permission. When we give ourselves to God we become His properties. The enemy does not dare tamper with God's property.
1 John 5:18 - We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.
Sin grieves the Holy Spirit. It is one of the easiest means to attract the enemies. As people of God we ought to depart from sin so that we don't grief the Holy Spirit and give the enemy access to us.