If the person who leaves would have been the big blind the next hand, then the next player is the big blind and it just moves around the circle.

If the person who was the big blind, and would now be the small blind is knocked out there are two different rulkes you can use --

Dead Button:, you just play without a small blind the next hand. The button then stays where it is for one hand (dead button) and the next hand everything gets back in order. (this appears to be the rule used in Las Vegas)

Advancing Button: you play one hand with two big blinds and the next hand you have the button post a small blind in addition the next players post there small and large blinds. (This is the rule we played with at Turning Stone)