We are currently playing the original module Keep on the Borderlands. The module has pretty clear instructions about removing any humanoid that is killed in the Caves of Chaos from future encounters:
When monsters are cleared out of an area, the place will be deserted for 1-4 weeks. If no further intrusion is made into the area, however, the surviving former inhabitants will return or else some other monster will move in. For instance, a thoul might move into the minotaur’s cave complex (I.), bringing with him whatever treasure he has.
And, for the individual lairs, here for example Kobolds, Orcs, and Goblins:
DM Note: Kobold losses will not be replaced, though injured kobolds will heal.
DM Note: Orc losses cannot be replaced, but after an initial attack by adventurers, the males at location 10. will move four of their number into area 9., arm these orcs with crossbows, and lay an ambush for intruders
DM Note: Goblin losses cannot be replaced. If they are being soundly defeated by intruders, the goblins will attempt to hide or flee east
Now, the goblin lair also has a wandering goblin encounter:
The passageways here are very busy, and for every 10’ distance covered by the party there is a 1 in 6 chance that they will encounter a group of goblins (see below) Check each time the party travels 30’ (a 3 in 6 chance) until wandering goblins are encountered, then check no further. When an encounter occurs, the entire bunch of goblins will attack and cry out an alarm (Bree-Yark!) at the same time. Wandering goblins are in addition to those found in numbered areas.
Wandering Goblins: 6 males (AC 6, HD l-l, hp 3 each, #AT I, D 1-6, MV (20’), Save NM, ML 7). Each will have d6 silver pieces. (They are patrolling and carrying messages back and forth. The group will also be carrying several bags (d6) of fairly good foodstuffs - not worth much, but quite suitable for human fare.)
Because of the "then check no further" I think there is but one single such group, that, when defeated will not re-occur. However, the entire wilderness area around the Caves of Chaos also can lead to encounters:
Nothing will bother the party when camped outdoors, unless they are within six squares of a numbered encounter area. [...] Treat otherwise as a normal encounter.
No details for these encounters are given, so I made up my own encounter tables based on the types of creatures found in the caves. However, should monsters encountered in these encounters also be removed from the limited contingent of monsters in the caves?
So far I treated it as the goblin patrols in the goblin warrens being one single group that is not regenerated (and forming a new one from the remaining goblins to go foraging), but the wilderness encounters as unlimited, based on the idea that these monsters are scouring around in the woods in multiple groups, and those are in addition to the ones in the caves.
(My players, who made the mistake to several times camp out to close to the caves at night and had ambush encounters, have started to question the amount of humanoids issuing from those caves...)