I was watching a documentary on the hubble and the commentator mentioned something about the telescope using reaction wheels and gyroscopes to point itself and then keep pointing in any particular direction.

I know that gyroscopes have a stabilising effect but can they really exert force to move the scope in the right or wrong or even any direction?

I thought that if say a wheel is used to create a turning force on the scope then when it was brought to rest it would have the opposite effect and we would be back where we started.

How does this kind of system work?

Yes try it with a bike wheel or similar. Hold just one side of the spindal and spin it by hand. Make sure you have plenty of room though as you will find it begins to try to hold itself level as you turn in a circle one way and pull down the other. If you turn faster it pulls harder

conservation of angular momentum is the law which is used.