And what is this supposition based on?
Are we going to ignore the thousands of examples that can be found where supposed spells, incantations, enchantments, etc. are written down yet nothing out of the ordinary has happened as a result?
As of yet, no line of causation has been shown to exist between things of this nature and their intended consequences (in other words, they don't actually do anything). Therefore, to assume something supernatural WILL happen when using something like a Ouija board is to afford unearned reliability to the prospect.
Until something supernatural actually does happen under testable conditions (that is, conditions that allow no tampering or other tricks) there is no reason to believe anything will.