There are two kinds of monsters in the game:
The first ones will never vanish. Once activated, they will "see" you once you enter their map and they will follow you from any distance. I suspect the game, that all monsters prepared for you on the map from the beginning of the game belong to this kind.
The second ones will vanish soon after losing your traces. All monsters that get generated while you are exploring a map or a dungeon belong to this kind.
Therefore, the four salads in the Ishad N'Ha area will never vanish and, once they find you (or rather, once you find them), they will always be after you. The same is true for spiders in Valeia, for the nymphs near the lake or for the rubble of trolls on your way to the lake. On the other hand, you can flee from randomly generated trolls or bats in Valeia or from randomly generated spiders in Ishad N'Ha.
Playing the game without killings can be a real pain, there are too much monsters that belong to the "permanent" ones - in all dungeons and on all maps. I've never tried it because it contradicts my play style completely
Nevertheless, you can become invisible, your invisibility is long lasting and it isn't cancelled when you attack or touch some being ...