Glomung and deeezbeats, you guys are missing the point. I didn't say demons don't exist or don't do anything. I said that things like Ouija boards, tarot cards, spells, incantations, etc. are ineffective. Could a Ouija board be used by a demon to influence a person? Sure, but no more so than a doorknob or Big Mac.
And that's my point. There's no more reason to fear or be suspicious of Ouija boards or tarot cards than there is of doorknobs or Big Macs. As soon as you start treating these things differently, you've given them power that they don't otherwise have. It's just like if I believe that every time I touch a doorknob, a demon is using it to enter my body, so I never touch doorknobs, campaign to get them banned, and tell everyone that doorknobs are evil. All that does is give an otherwise inert object a power over me that it never really had.
It's the same as what I said about the first Commandment earlier. God didn't tell us to not have any other gods before him because there really are other gods. He told us that because if we start worshiping another god, we would be giving an otherwise non-existent entity a power over ourselves (the power to move us away from God) that it never really had (because it doesn't exist).