There have been no critical hits tables released by WOTC. When you get a critcal hit, you do either 2, 3, or 4 times the damage depending on what weapon you are using.
The reason for not having critcal hit tables is exactly for the reason your thread title suggests. They make it way to easy to take out opponents (either by the players or the DM) with a single attack. This is also why there have not been any rules for called shots.
Basically, if you do enough damage from a critical to kill the opponent, then you can use your imagination and describe how they died. For example, instead of just saying the orc died from your blow, instead say "with a mighty swing, the orc falls to the ground decapitated" or whatever you want to imagine.