I have a glitch that works to my advantage. Every time I start up a game, it's at the waxing crescent point of the lunar cycle. If I play for two Sim days and exit before the evening of the full moon, then I don't get zombies at all. EA will probably fix this eventually, but for the moment, it works.
For people who don't have the glitch, going off the lunar cycle and setting it on a gibbous moon should eliminate zombies except for the ones created by potions.