Hair pulling is another way of reducing anxiety, and is known has an anxiety disorder. What may begin as simple pulling of the hairs on the face including the scalp, eyelashes, and eyebrows, can turn into pulling the hair to the point of baldness.
As with any other behavioral disorder you need to get to the root of the problem. You cannot just ask them to stop pulling their eyebrows out, pulling their hair out or pulling their eyelashes out. This is a response to physical or anxiety issue, not that they actually want to pull their hair out. It's why you cannot just begin by moving a child's hand away from their face or scalp. You need to find out the reason for the anxiety before you can stop it completely.
In order to get to the bottom of hair pulling the first stop should be your physician. If it is concerning a child, make sure it's a reputable pediatrician. They have found that there are some abnormal brain metabolisms that can lead to hair pulling and it's important that you rule these out for you begin treating for anxiety issues.
Once you've ruled out any physical issues concerning hair pulling, then it's time to look at behavioral therapy. Again, learning as much as you possibly can about the psychological approach to trichotillomania can actually help with the treatment. Knowledge is power and with behavioral therapy it really means it.
The thing to realize with hair pulling is its not a normal function, it is a result of either a physical disturbance, or a mental one.