I get that it's "unstoppable" but the whole point of an 
ult is to either push enemies to your team or disengage a terrifying wombo combo. Right now, you can only ult after getting wombo comboed. Meaning your ult is useless already because you are probably the only champ alive on your team. players should be rewarded for fast reflexes on blocking a super fast 
ult.
Villiers3 votesAzir players are rewarded enough with their idiotic kit. Ban malphite, flash away from his ult, don't allow him to ult your entire team.. Problem solved.
Virgin Ray1 votesdon't allow him to ult your entire team
Which is why ult should block ult.
Shac Norris2 votesNo.
It's UNSTOPPABLE, your puny line of soldiers ain't gon' block a mountain.
Mecha MaIphite1 votesor you could not clump up in a tiny little space
Villiers1 votesSpoiler alert.
It's not league of Azir, bro.
FioraWillCarry1 votesSpoiler alert.
It's not league of Azir, bro.
Says who ?
destos2 voteswhat part of "unstoppable" dont you understand?
Virgin Ray1 votesI'm saying it shouldn't be unstoppable for ult. If not, it shouldn't be unstoppable at all.
OurLestrade1 votesYou answered your question in the first sentence chief.
MorganFreemanBot1 votesAzir's ulti explicitly prevents dashed.
Malphite's ulti is explicitly immune to being stopped in any way, shape, or form.
Basically it boils down to which 'rule' is more explicit. Azir has a clause of "Prevents all dashes", which means dashes. Things like Lucian E, Lee Q / W, and various other dashes which can be prevented in other ways or do not have any specific protection. Malphite's ulti, on the other hand, is a dash which explicitly mentions that it's immune to any effect which would stop it. It has the more specific permission, so it takes precedence.
Basically look at True Damage and its interactions with immunities. True damage says it cannot be be mitigated, reduced, or avoided... But abilities which explicitly state that they prevent all damage (Fiora's parry, Kayle ulti) have a more specific ruling in that they prevent all damage for the duration.
Ozag the Mighty1 votesIt blocks trists jump...is that a dash???
Ruin Lance1 votesbecause he Can't be disabled!, sir