You push the ball long, and the opponent does a slow but spinny loop. And you block it off, over and over, and can only stare at your racket in frustration. This is often the bane of every beginning and intermediate player. They know they have to aim lower, and yet, the next time they face another spinny loop, they still block off the end. Why?
It's simple - a player does what he practices. And the huge majority of your blocking practice is likely against either players with less spinny loops, or who are looping against your block, and so have less spin than one against a backspin. And so your natural instinct is to block as if the ball has less topspin – and so you go off the end.
First, the basics. To block a spinny loop, you must close your racket more than you would against a less spinny loop. Your instincts may tell you to do one angle, but you likely have to close it even more, perhaps aiming for the net, perhaps even the bottom of the net. Give the ball at least a light punch - that way the spin won't take quite as much on your racket. Once you’ve made one good block off this spinny loop, remember the feel and the contact, and repeat. (It might also be helpful to watch top players block against spinny loop – the visual image of how effortlessly they do it will help.)
Now that you know the above, it's easy to block spinny loops, right? Wrong. You have to practice it. And that means finding someone with a spinny loop so you can practice against it. And the best way to practice against it is with an improvised multiball drill. Get a bucket of balls for your partner. He serves backspin; you push it back; he loops; you block. And that's it - you DON'T play out the point. As you are blocking, your partner should be reaching for the next ball. Result? He gets lots and lots of looping practice, and you get lots and lots of blocking practice, and specifically against spinny loops. As you get better, block more and more aggressive, and as you improve, perhaps practice counterlooping or smashing them.
Once you've mastered this, the next time you face that spinny loop in a tournament, you can become the bane of your opponent!