Gainsay

gainsay \gayn-SAY; GAYN-say\, transitive verb:
1. To deny or dispute; to declare false or invalid.
2. To oppose; to contradict.

"The evidence was clear; there was no way that the family could not possibly gainsay their presence at the scene of the crime."

Someone who gainsays is called a gainsayer.

Gainsay, you will; gainsay, you won't..