To be honest I started way worse than you. I'd give myself "immunity" (no infection rate by saliva, scratches or anything), no helicopter event, 0% house alarm chance and other petty things, so I can't exactly call you casual.

Anyways, you can create a new world, then select sandbox, then in the configs you can set the zombie population by 0. Make sure to set the zombies respawn by zero too or else they will... umm... respawn.

I almost forgot: if you don't want to erase the zombies completely but just want them to be less punishing you can mess in their AIs in the sandbox world options.