I’ve had similar problems. I don’t mess with the spawn rates but four building towns still routinely spawn 60+ zeds. I remember going into a town with approximately six buildings in it, and fighting about 200 of them just to get past the first two buildings.
They typically pile up so much that by the time I get around to pulping/butchering, the first waves are getting up again and I can’t pulp/butcher immediately after killing them because there’s always 3-5 more that notice the character and start approaching in the middle of the process. They’re never hard to put down again but its always a chore. Seriously, I didn’t even want to go into the town that badly; all it has is four houses and sixteen freakin’ liquor stores. Who drinks that much?!
Jokes aside though, other than Shard’s suggestions, people have said that the Chain Lightning CBM is good for crowd control. It arcs to all enemies nearby to the one it hits, but it can hit you if you are too close so its probably not good for indoors. In my experience it doesn’t do much damage so you’ll need ample reserves of power to actually kill large numbers of zeds, though.
You can make Molotov cocktails from Glass Bottles, Rags and Gasoline. They create a square of fire either 3x3 or 5x5 (can’t remember which) about three turns after throwing them, and go out instead of exploding if you don’t throw them, but immediately after you light one you will remain lit up until you throw it so if you light it up at night then all zeds in visible range will see you, and rain seems to diminish its effectiveness. Also, since it’s fire, all burnable drops will be damaged and destroyed. And I think ammo will explode when burned, so if one of them drops some its better to be far away.
Acid Bombs can be made from Concentrated Acid, which can be created from batteries, and a few other things. I’ve never used them so I don’t know how much area they cover.
Grenades can be made from 00 Shot, and a few other things, but loud explosives might draw more than they kill if hordes are enabled.
I, personally don’t have a crowd control solution; it’s always a slog beating down the hordes, butchering a fifth of them, then rinsing and repeating. By far the best method I’ve found for killing large numbers of zeds is simply to run them down. The vehicle parts that Shard mentioned (the Shredder and Rams) will increase the damage output of ramming into a creature. This will not stop them from reviving though; you still need to pulp/butcher manually, unless you use the mod that disables revival. You’ll also need to repair the Shredders and Rams periodically, as well as your car. Headlights are typically fragile, so they should not be around where the Rams are or they will most likely be destroyed.
If you’re short on parts like the shredder, mounting spikes (a level 0 fabrication recipe, so you most certainly have it) actually has some of the highest impact damage output I’ve seen. They take a very short time (one or two minutes each) to make using scrap metal or chunks of steel, and roughly 36-40 minutes each to mount at 6-8 Mechanics. The benefit of equipment like the shredder over just mounting spikes is that the description of the shredder states that it is designed to pop off rather than break, where spikes can be destroyed and rendered unusable. Spikes will also need to be repaired periodically and are less durable than shredders and the like, but the ease of replacement and ridiculous damage output makes them well worth using. I’m not sure if it is exclusive to a mod or just the Hulks, but I’ve installed spikes, then rammed Hulks at 30 MP/H (or 48 KM/H) and they exploded into chunks of tainted meat instead of dropping dead, so I didn’t have to butcher anything. That doesn’t seem to happen to any other creatures, though. It scared the crap out of me the first time it happened because I knew I had just killed a Hulk but couldn’t find the body.
If you’re worried about your primary vehicle being damaged, you can always mount the spikes onto a temporary one just laying around town, and if you have a Remote Control Unit CBM or whatever electronic device (I think its a control laptop), then you can install an electronic control unit into a vehicle and sit in a safe, reinforced area and run the hordes down with impunity, so long as you don’t take the vehicle too far away and lose control of it.
Most importantly, if you’re using a vehicle that has wing mirrors, take them off before you go running things down, because a few zeds will slip by and they will hit the wing mirrors and they will knock them off and if you run over them they are destroyed. I’ve lost so many mirrors this way that I habitually steal them from every single vehicle I come across in every city I go into because I know I’ll inevitably need replacements.
Out of curiosity, what’s your current kill count?