Title.

I have seen somebody do it but I don't know how to. I have tried casting another ability or moving but it won't work

Have the guy Shen wants to protect killed.

If you are shen and cast ult on someone and want to cancel it, try to push S. It should stop actions. Not sure about channels tho. But give it a try.

Tried to push S but it doesn't work on this

only flash can cancel it

walking doesnt stop it?

getting stunned/silenced/knockuped by your fellow lane enemy will cancel it.
Ask them if they could do it for you when you ult next time.

but karthus ulti can break it, np after death or
idk exacly but moving breaks it too, i think