Stopping the slide will NOT prevent the gun from firing! The slide doesn't move until AFTER the bullet fires. A hand on the slide when the gun fires is likely to cause a malfunction that prevents a second round from firing, but has nothing to do with the first. Unless...
Now, if you manage to grab the gun and MOVE the slide just a bit, the gun (most newer anyway) may not fire because it will be "out of battery" and can't fire. In fact, there is a safety called "out of battery" that prevents firing.
( see
http://www.snipercountrypx.com/pm-1502-158-walther-p99-qa-military-4-inch-9mm.aspx )
If you're hand is over the slide when the gun fires, you're in for a nasty pinch and a burn. Adrenaline may or may not cause you to not notice right away though.
Watch a high speed film of a gun firing in dim light and you'll see the reason for the burn.
Blocking the hammer in an a-la Hollywood move is an unlikely proposition, and in the heat of the moment, I don't want to be looking to see if a gun has an external hammer to start with (Glock for example, has an internal hammer).
To throw another wrench in the works, lets not assume that we are not facing a revolver. Whole different game then.
One more, revolver or not, a snub nose (38 or baby Glock) doesn't leave much to grab & use for leverage against the attacker.
So many things to consider...