If you did in fact mean Hula, then what's stopping them is the fact that ISPs don't really care what you twirl around your hips as they're unlikely to make a dollar from it.
What's stopping them from blocking netflix is that they don't want a revolution on their hands. Hulu was their attempt at vertical integration, they are undoubtedly aware that hulu is not a competitor enough though to topple netflix. Attempting to say their customers couldn't use netflix would cause them to lose a huge chunk of their customers.
As for pirate bay, they probably realize it would be down for a matter of hours at most, and that trying to block it would create more problems and waste more money than would gain them.