People who genuinely think they're psychic, but cannot prove it fairly, could be viewed as not deceitful, but misguided.
People who knowingly employ tricks to achieve the illusion of psychic ability could be viewed as not misguided, but deceitful.
The latter group are usually far more convincing, and so they have a greater influence on people who may choose to believe in such things. The believers in psychic ability see these tricks as firmer 'evidence' of psychic ability than the readings often attempted by the misguided "psychics".
I don't think you can deal in absolutes and say one is wrong and one is right. Similarly, the ethics involved is fairly subjective; some of the con artists merely see their work as harmless illusions, choosing to believe that the people who then subscribe to artificial beliefs are in some way naive or stupid - they don't see themselves as responsible for converting people if that happens. Similarly, some of them think it's best for their illusions if genuine psychic ability is believed in, so they can get away with more...but again, probably don't have ethical issues with that, claiming once again that people have the opportunity not to believe.
My personal solution would be to explain to the people that believe in psychic ability that there is no known scientific evidence yet...and explain the importance
of evidence.
If you're interested, this is interesting reading:
http://derrenbrown.co.uk/blog/2011/10/testing-psychics/" target="_blank" target="_blank" target="_blank
There's my $0.50.
As for suggestion...it depends how it's framed. If it's claimed by the medium that the oujia board is actual communication with spirits, but in actuality the medium knows about suggestion and believes that it really works through suggestion, then that is dishonest and endorses the belief of contacting the dead. If on the other hand it's claimed to be suggestion, then, well, no real dishonesty going on there at all - in fact, that's quite fair. For the medium that believes the oujia board is genuinely psychic, then fair enough - they might be right (as you can't really turn round and COMPLETELY disclaim the possibility) but it's highly, highly unlikely. So they're very probably misguided, and it's very probably suggestion, in which case we go back to the whole idea of their beliefs being challenged. Of course, these people will be usually reluctant to challenge their beliefs...or at least do it in a very biased way. Not always, but usually.