Our strategy certainly involves plumbing, the most reliable source of zombie clearing within the game. A hero with the shopping cart and a utility belt has 8 free slots available. For very short range clearing, you can have 8 pistols (24 zombies), for medium range clearing you can have 7 pistols (21 zombies) and for longer range clearing you can have 6 pistols (18 zombies). This is under the assumptions of 6 AP, 12 AP and 18 AP. The range increases with the copletion of Watchtower lvl 5.
The above number can increase with the finding of the Aqausplash gun (5 zombies). The plumbing also allows for scavangers going even further out to take water bombs with them, allowing for zombie kills without using a valuable bag space as the water bomb vanishes upon use.
Next comes how many zombies to clear. You need to have a good understanding of how zombie propogation works to do this efficiently, which you can find here How does zombie spawning in Die2Nite work?
So, depending on how many zombies are killed in each zone relates directly to how many you should kill to have the square clear the following day. Example a square with 2 zombies , you would need to kill both for it to have 0 zombies the next day if it was not surrounded by other zombie filled tiles. However, for a square with 8 zombies, if you kill 6 of them the 2 left will die during the nights attack.
Finally, is how many to clear across the whole map. There are two different strategies you can use for this. You can either keep a low number of zombies across the board, with the occasional high zombie square to keep the minimum number of zombies on the map up, however this can be a very tricky situation to judge and the zombies would re-spawn very quickly as every zone has zombies in it still.
The strategy I feel is best is to leave your smallest replenishment area with no useful buildings in it full of zombies. You can then clear the rest of the map completely, as long as the total number of zombies in the one area is high enough to avoid the mass respawn scenario. So if you have a very small zone with no buildings in it, that is an ideal area to leave as a zombie "zoo". Each time this one area replenishes it requires you to arrange a very large group to travel together and deplete each zone as you pass through it. Eevery other zone that replenishes every person who goes out can do so safely alone! It also means each day you only need to clear a small number of zones sarrounding your "zoo", and not the whole area that has just replenished.
This game has many different strategies for a lot of things that do work, but its all about opinions and effeciency. And certainly in my opinion, this is the most effecient strategy by far.