Nunu's ult is a channeled ult, which means he must stay still to use it. When he moves it ends.

Janna's ult acts as an interrupt meaning it can stop channeled ults.

Malzahar, galio, nunu, katarina all have channeled ults.

Janna (ult), poppy dash, trist (ult), alistair's headbutt are some examples of interrupts.

These ults can also be stopped by stunning or silencing.

Hope this helps

Other channeled ults I can think of right now are Miss Fortune, Warwick, Karthus, Pantheon, Shen, Fiddlesticks (charge up), and Janna

Good additions!

So we have:
Miss Fortune, Warwick, Karthus, Pantheon, Shen, Fiddlesticks (charge up), Janna, Malzahar, Nunu, Galio, Katarina

^all have channeled ults that can be countered by an interrupt such as moving them, silencing, or stunning.

It is also worth noting that anivia's ult persists while she moves, but ends Shen she is silenced or too far away from the circle

Ezreal has a channeled ult but it can't be canceled (due to a bug).

It's not that I don't think Ezreal's ult can be cancelled, it's because the wind up is one second, so typically by the time you see it and commit to stopping it due to lag, human reflexes, and the time it takes to target Ez, you cc after the channel is finished.

Although I could be wrong, but that's what I always thought

Pushpushpushpushpushpushpush

Nope. It disrupts it, and it is "broken", so it doesn't happen. BUT! He has to wait for the cooldown. You can also taunt him or stun him during his ult to stop it, before he activates it