I've been waiting for Riot fixing that, but after like 5 patches or so I don't know if they're not aware of this, or it's an intended (and very annoying) feature. The thing is that you can't cancel the channel like it is with many spells that have long channeling time - by right-clicking. You can cancel by using W tho, but honestly I consider this one a bug.
It can be very annoying in some situations, for example when you see Pantheon's/Xerath's/Jhin's ult indicator right after you started channeling, and you have no chance to escape, because the channel has to end on its own. Or when you see a global ult like Ezreal's coming you way, and you can't dodge this, because you've just started channeling, so you just stand there like a moron and get hit.
So my question is: is this thing intended, or not?