Wool Block

Wool Block is when a rabbit ingests fur while cleaning itself.
It can also ingest wool and hair if the hair is allowed to shed on the rabbits food pellets, water dish and hay.

Replace any food or water that has hair in it.
Stomach acid can't break down hair therefore it forms a ball in the stomach. These balls can get quite large and eventually the rabbit will loose interest in it's food and water.

Smaller blocks can travel into the intestines where they become lodged and cause a blockage. This is very painful for rabbits.

Symptoms

Signs of a wool block are lack of appetite or refusing eating anything. You may notice the rabbit hunched up in the corner of the cage, possibly grinding it's teeth in pain. They'll have no interest in coming out of the cage to play.

Treatment

Fresh pineapple and papaya tablets
I do recommend this treatment. Many breeders feed fresh pineapple and papaya tablets to prevent and cure wool block. These fruit contain enzymes that help break down the wool block. The tablets can be bought at rabbit shows and on-line websites like the "Rabbit Medicine chest".

Hay
My vet advised me to stop feeding pellets and feed Timothy hay while trying to dissolve a hair ball.
The pellets have stuff in them to bind then into a pellet form, the same stuff may bind to the hair ball and make it larger. You can also feed parsley, and critical care bunny food. Critical care is made by Oxbow.
Check their website for distributors near you. It cost approx $20.00 for a small bag which goes a very long way.

Cat hair ball medication
Other breeders use Cat hair ball medication. I don't use this method since the theory is to coat the wool block in a petroleum jelly so the rabbit can pass the wool block. 
The trouble with this is if the wool block is too big to pass it may become more firmly lodged in the rabbit. Then the mass is coated in petroleum jelly and nothing you give the rabbit will be able to dissolve the wool block or penetrate the petroleum jelly coating it. Still many breeders swear by it.

Hot water bottle or heating pad
I use a hot water bottle and lay the bunny across it. 
The bunny will fight me for the first few minutes but when they realize that they feel better they will then settle down. 
After they are on the hot water bottle or heating pad I'll notice that their hard stomach will soften up. 
I carefully massage their stomach. 
Being too rough can cause pain and possible injure the internal organs. 
I also gently stretch the bunny out and then back to a sitting position a few times. 
Then put them stretched out on the water bottle again. 
I repeat this and often they start to pass gas. 
Then I let them run around the house. 

Bunny Enemas
I've also given them bunny enemas. 
I boil water in the kettle and let it cool to baby bottle warmth. 
Just warm enough that the rabbit doesn't notice you're giving him an enema. 
I put a cupful of water and add a TBSP of vegetable oil. 
Stir and load a 15cc to 20 cc syringe (no needle) full of water. 
I wrap bunny in a blanket to immobilize him and then slowly push the water into it's anus using the syringe. 
Hold the anus closed for a moment so the water will travel upward instead of out again. 
Then let bunny run around. 
You can repeat this a few time. 
You should start to see dry poop on the floor within in a few minutes. 
If you don't see any poop it may be a block much higher up. 
In which case you can keep trying the water bottle and message but the bunny may not make it. 
Look up the term " GI Stasis " on the web. 

Before and after care.
I feed the bunny a Probiotic paste such as Ben-bac, Pro-bac or a horse probiotic paste (for colic) found at a feed store or tack shop that sells vitamins etc. 
This will replace the good bacteria in the bunny digestive tract. 
Once they stop eating and pooping the good bacteria dies and the bad toxic bacteria like e-coli thrive. 
The probiotics will restore the good bacteria. 
I feed parsley when I feed the probiotics. Parsley is a high fiber plant and I've never had trouble feeding it to baby bunnies when I need to get them eating. I also feed Timothy hay. A good hand full in the morning and a handful at night.