This is when a rabbit ingests too much fur from cleaning or pulling
and it causes a blockage in the stomach

Symptoms

Stops eating or eats less
Fur imbedded in feces
Size and/or number of droppings decrease
Sometimes blockage can be palpated.

Treatment

Hair Blockage is more common in long haired breeds such as Angoras
however all rabbits can get this.
It is better to prevent this by grooming and providing your rabbit with hay.
Treatment in severe cases is surgery which is not always successful.
As soon as you notice signs, you can try a hairball remedy
such as petromalt or a treatment of 10cc's of fresh
pineapple juice once a day for 3 days.
There is an enzyme in the juice that can break down the mucous around
the hairball and helps the rabbit to pass the substance
forming the block.
It is important to move the blockage and get the rabbit eating as soon as possible.

Prevention:
Exercise
Timothy Hay daily
Grooming especially during a heavy molt