You can't (or at least I couldn't) cleanse a malzahar ult. However, you can QSS it. QSS reads "removes all debuffs," whereas Cleanse reads "removes all disables and summoner spell debuffs affecting your champion." To me, it seems clear malzahar's ult is a disable, and should be removed by cleanse, hence my bug report.
Cleanse has no effect on Suppression, knockup, knockback, pull, and self crowd control.
malzahar ult = supression.
Champions with Surpression -
(All on their Ults)
Recommend a 
next time you face Malz.
For more info on helping to remove certain types of CC please visit -
http://leagueoflegends.wikia.com/wiki/Types_of_Crowd_Control
As in you couldn't remove the suppression, or you couldn't stop the channel full stop? I'm pretty sure the damage part of the channel has always continued after cleansing or something like that :O
cleanse has never removed supressions totally intended sir
