Press C and enter sneak mode - then you can see in red how close you can go. If you are outside the red area then they can't see you. If you go to the edge of the red then you'll be able to get them.
This is how to kill them. Not sure it warrants spoilers but you never know.
To destroy the Shriekers you must use Source Vampirism - not the basic wand attack.
Equipping a purging wand will give you the skill on your skillbar (4 uses only) but it will only work if you have your collar removed. Once you have used up the 4 you'll need to use the other wand or recharge it.
