We love to play with things in the spirit world. Some don't believe in the spirits, nonetheless, it does not diminish the reality of what can happen when you play in this realm.
The premise behind the Ouija board is seeking answers from the spirit world. This practice is often referred to as divination. In the Christian faith, this is prohibited because we are to seek answers only from the Holy Spirit (God's Spirit) for direction and guidance.
If we seek "another spirit", it would presume to be an evil spirit. In Jesus ministry, as recorded in the gospels, we see him "casting out" evil spirits from people. These spirits were very harmful and most often coupled with sickness and disease;:
The evil spirit shook the man violently and came out of him with a shriek.
When Jesus saw that a crowd was running to the scene, he rebuked the evil spirit. "You deaf and mute spirit," he said, "I command you, come out of him and never enter him again."
For Jesus had commanded the evil spirit to come out of the man. Many times it had seized him, and though he was chained hand and foot and kept under guard, he had broken his chains and had been driven by the demon into solitary places.
Even while the boy was coming, the demon threw him to the ground in a convulsion. But Jesus rebuked the evil spirit, healed the boy and gave him back to his father.
And we see the same ministry carried out by Jesus disciples:
Crowds gathered also from the towns around Jerusalem, bringing their sick and those tormented by evil spirits, and all of them were healed.
With shrieks, evil spirits came out of many, and many paralytics and cripples were healed.
For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms.
I would recommend these hubs by Dr. Pastor Carlotta to understand some of the effects that these evil spirits can have on a person- even if they believe it is all in a person's head.