In the greater reality, there is only God and nothing else. Everything has its existence in the Creator. God is light, love and perfection, and out of light, love and perfection come only light, love and perfection. There exists no opposing force of evil and/or darkness. What is not God is void. Empty, black, powerless, nothingness.
While it is possible for another spirit to possess your body (with your permission only); it is not possible for another spirit to posses you. You are not your body. You will always be you. Safe and secure in the Oneness of your Creator.
Intense thoughts can produce form. It is possible to fear something so much that you create for yourself that which you fear the most. People can fear devils or demons so intensely that they create them. These created monsters are thought forms of fear and will vanish at the first sight of Love. However real they may seem, they derive their power only from fear. They have no power over anyone who does not fear them.
In order to avoid "negative thought forms" keep focused on God's love and compassion for you. Know that you are safe and secure at all times.
There are no such things as actual devils and demons in the greater reality. Nothing to fear at all. But for those who insist on being afraid of something, I have created a "dark side" web page so they may be scared. It would be especially good at Halloween. I hope you enjoy it. :-)