The Unthrower

[Crystalion]

There are only three effects that can stop monster heal regeneration:

1): Prevent Monster Heal
You...

2): Open Wounds
You...

3): Poison
You...

This is a good explanation, and concise, but allow me muddy things up by adding...

Iron Golems, Valks, Shadow Masters (and warriors, if emulating your Venom) and Skeleton Mages of the poison variety are all capable of using some of these methods for "you" to stop monster regen. The Iron Golems, of course, can be made from an item with any of the three methods. Valks and Shadow Masters get rolled equipment, which could have any of the three methods.

In addition, Mind Blast, Conversion, Attract and Confuse all have the potential, in some cases, to cause poison (or poison from Spectral?) to be applied on monsters.

I may have forgotten something, but I think you can see that little in D2 is as simple as it first appears. :-)

p.s. in addition the implementation of the game frequently causes monsters to not be regenerating that "should". Typically once you harm these monsters again they do begin to regenerate. (Leaving zones so that monsters stop getting processed is a large part of triggering this effect).