How about making it like hockey - call penalties and not fouls.The hope is that people would play clean. Think of a pick-up game - if you go fouling all over the place, people get pissed. Generally, people play clean and play the ball, not the body.
Guy gets sent off for say 2 minutes - side has to play with 10 men for that short period. Maybe make it 4 minutes due to the running time nature of soccer.