As Mr. Leach has already pointed out in a comment, the most obvious word to use for this purpose is deny. Oxford English Dictionary, "deny, v.," definition 2:
Logic. The opposite of affirm; to assert the contradictory of (a proposition).
(This may be so obvious that it is arguable that the question should have been closed. Given that it hasn't been, and that it has already received a couple of other answers, this obvious answer needs to be posted as such.)
