Allowing debate to occur over the Halocaust negates not only its entire existence, but also the suffering of millions of people. Irrefutable proof and eyewitness accounts of thousands of people provide enough facts to support the tragedy's occurence. No further debate is necessary and it only causes further pain and suffering for those involved.
Allowing debate over holocaust denial allows people to get their viewpoints out. Those who believe the holocaust did not happen just may be lacking facts. If they are allowed to debate the subject they may realize that their beliefs are just not true. I don't think not allowing people to debate over the holocaust would help matters.