I too have unpopular opinions of oujia boards. I think it's a tool, like anything else. I think it's something that needs to be respected, and used with care. I don't have one myself, but a friend of mine uses one from time to time.
He had some bad experiences early on, when he was using it with friends who made fun of it and did it while drunk... and that's stupid. Using a divination tool while drunk is ASKING for trouble.
On the other hand, when he uses it with people who are clear about their intent to speak to good spirits, cleanse the place before and after, and use a circle of protection, he's had NO problems. In fact, he even used the board to get rid of the negative beings that accumulated after early unfortunate uses of the board.
His first experience was at a party, with drunk people, as a party favor, and they did have poltergeist activity. He went from atheist to pagan because of his experiences with the board.
So yeah, ground and center. Make sure everyone present does that. Cleanse the area. I think that intent to talk to beings and see the beings, as well as the fear of the Oujia board itself, can manifest into critters that take up residence is what happens with the people who have those bad experiences. As long as you feel that they won't go away until the board does, well... that's what happens. Also, it's possible that those "things" were there all along, and one didn't notice them until you used the board and got more sensitive to one's environment.
Well, that's my theory, my less than two cents.