You cannot debate the deniers because their entire point is to use a debate with you to further advance their ideas.
A better strategy would be to meet them with Ridicule and Mockery.

EG

David Duke is saying these things in order to deflect attention to the fact that he molests mountain goats. (or something equally outrageous.)

Etc

Now it is he who is on the defensive, and he will never be able to advance a denier argument without putting a companion idea (accusation of mountain goat molestation) into the head of his audience members.

These people will go away faster in the face of laughter then in the face of indignation.