With The Warwick Rework, CCing Him During Ult Should Stop The Damage
Supposedly this was a "buff" he received ages ago to make him more viable.
Now that he has two dashes, a fear AND his suppression, however, Warwick feels a little overbearing when you can properly predict his ult during a close range fight, throw down a knockup / stun / whatever, have it successfully land, and still continue taking the full damage during it.
Malzahar, by comparison, CAN be interrupted during his ult and the damage will end. Not sure why Warwick's ult doesn't work the same way.