Is it considered a denial and defiance of Islamic law for a person to say "no" in response to the question "Do you mind doing such and such?" if the act is religiously forbidden?

The questioner is suffering from obsessive-compulsive disorder (waswasah), and he must be kind to himself and ignore the waswasah.

It is obligatory for one who is asked about an unlawful act to show abstention both in word and deed. If he does not resist, he sins, but he does not disbelieve. This has no relation to the disbelief of denial (kufr al-juhud) and does not necessitate defiance of the Sharia.