I played L4D2 online with people that stop Hunters from getting them almost always. Instead, they get me 7-8 out of 10 times. How can I improve my chances to block hunters?
You can melee a hunter mid-leap to cancel its attack and stun it. It requires some practice on the timing to get it perfect, but it's extremely effective, especially since the hunter is so loud before it pounces. Learn to identify the different sounds each infected make so you know what to prepare for.
After you stun it, you can deal with it however you see fit. If you melee it enough times that will kill it too, and is a satisfying way to taunt hunters who get too close to you.
If you mean a dead stop (melee as he's pouncing), then there is a trick: Hunter's box model (the wire-frame model responsible for hit-testing) is way before his rendered body.
So if you want to dead stop him you should time your melee just before it gets to you.
Whether the melee connects depends also on your ping to the server. Remember, hit-testing takes place on the server and server only on online games.
Also, bot hunters can cheat by clawing you while stunned, so make short work of them with your rifle as they are stunned and don't go in for another melee.
Update: This video demonstrates the difference between the hitbox model and the what-you-see model.