To turn off the zombies edit the file /Servers/Public/Config.json
Locate the following lines:
"Zombies": {
"Spawn_Chance" X.X,
And put 0.0 as value (default values vary depending on the difficulty level). You should have 4 occurrences to be replaced; giving you every time:
"Zombies": {
"Spawn_Chance": 0.0,