There is a possibility to block interrupt of Ult.
use OnProcessSpell to detect the successful ULT cast, then GetTickCount (new API func added) the current time.
In that time, block any moving/attacking.
Another way would be to check for the UltBuff, but that might still not be accurate to milliseconds.
How can we block any moving/attacking?
There is no api to disable build-in orbwalk.