 Just dedicate a minute or two before your session to state firmly you are not interested in talking to anyone who wants to scare or control you, and that if they try, you're going to tell them goodbye. Or, again, better yet, don't mess with spirit boards, Ouija boards, or Charlie Charlie at all. This might not sound like it would do much to deter a demon, but it's really hard for them to possess or oppress you if you're not giving them an opportunity to even be around.