How do I 'turn block'

Or when you basicaly attack a unit then where the owner attempted to move it it doesnt move...

I cant explain it

Basically you have to make your move "before" he moves the troop you want to block. So, if you want to block one stack of bombers of your enemy the first thing you have to do in your turn is attacking that stack with one unit that can reach it. So, if the first thing he makes is moving that stack you will have 50 chance of blocking it. If he doesn't move it first then you have a much higher chance (still not 100, because sometimes his second move can 'happen' before your first one, but it's not normal).

You will have a 50 chance of blocking IF there are only 2 people in the game.