Mod does what the title says.

NOTE! You cannot do "Marshal Law" achievement while this mod is active.
However, you can change one line from Lua source to enable Marshal units for Midland Ranch heist.

In main.lua line 5, change this line

td[level_id].ai_marshal_spawns_disabled = true

to following:

td[level_id].ai_marshal_spawns_disabled = level_id ~= "ranc"