I'm in support of population control, but I'm against total removal. If a tile has a population of 20, and you kill 50 zombies with on box of bullets, then the game is going to get awfully boring once the town is empty. I know that there's always more towns, but abandoning your base to go play the game won't be very entertaining.
A "Game of Life" style system might be best. A tile may be cleared via repeated killings, halting all spawns. For every neighboring zombie tile, an empty tile has a chance to regenerate by a certain amount. This way, it's possible to clear an area and build a safe house, but you still have to make regular patrols to keep the horde from encroaching on you. And to keep the game from losing all difficulty, you could still encounter roving horde events (events right now, not overmap movement) that would trigger a large wave of some 20-30 greens plus assorted specials as befitting your skill and/or what day it is. A roving horde event could depend on the cleared size. If you've cleared out 9 tiles to do a little base, then it's unlikely, but if you've cleared out 50 tiles and started reclaiming the town, then the roving horde event is more likely. If you have a small area cleared you're more likely to step 2 paces outside and encounter zombies, so a roving horde would just be a pain. If you've got 5 miles around you clean, then you're probably looking for excitement and wouldn't mind a bit of a horde to keep you entertaining. In future releases, hordes could be roving overmap entities spawning some distance from the player, perhaps lead by a smart zombie scientist and leading the horde along some random path and/or directly towards the player.
For the record though, I'm against total extinction. You need infinite spawns if you want to keep the game infinitely interesting, otherwise you get a "win" scenario, and let's be honest, no one is meant to win a roguelike.
NINJA: Soda is in cans, insects can't smell it until it's opened.