Yes. When triggered, it will affect as many creatures in its area until it has affected 150 hit points worth. If there aren't enough creatures with that many hit points in the area (or some save), it will remain active for 10 min./caster level and will attempt to affect any creatures that do enter until it gets its
pound of flesh150 hit points.
Can one create a permanent symbol of death for their hermit's hovel that keeps out unwanted wildlife by basically just being a ~90% drained version of the spell so that whenever an insect or other vermin shows up, it eats 2-6 of the remaining 15hp of the spell, thus never requiring bug spray or other repellants?
Yes of course you'll have to sacrifice ~135 hp of useless villagers every so often so it doesn't kill the few people you do occasionally allow to visit, but that it's the village's fault they ruined your spell by allowing their children to wander the forest unattended...