No problem according to Charlie Brooker:
"Pay someone bigger than you to make your knees fold the other way if you don't submit eight hundred words by five o'clock. You'll be surprised what comes out."
I suspect it works for any other "block" too.
But amidst the levity there's a serious point here. "Work expands to fill the time available" is an immutable law of the universe.
So get a deadline.
And make the penalty severe for missing it.
You'll be surprised what comes out.