Once you turn over a tape, you're at the mercy of many different links in the chain to it's final destination on your home TV.
There's the broadcast entity. The satelite link. The local cable provider. Your own DVR. Any one of these links is quite capable of turning your pristine footage into pixelated dog poo! Realistically speaking, they all contribute a little bit.
I watch on Direct TV. Some channels are better than others, but take a look at what happens in the fades to black or cross dissovles on most programming. That's where the "blockiness" shows up. I also Tivo all of our shows just to check and see how they were broadcast. Yes, it's painful to see how they look. One thing I've noticed is that you really can see the difference between DV footage and the higher res formats like DVCPro 50 or Sony's Xdcam. In the color correction suite, the DV holds up pretty well in comparison to the other formats. Once you get to the end of the chain however, the difference is noticable.
In conclusion: What can you do? Be satisfied that you turned over a master tape that looked good when you finisished it and met all tech specs required for delivery. Beyond that, short of handing over a DVD to every home viewer, you're at the mercy of too many independent entities to do anything about it.
Mark