This could be a huge problem and be a sign of many illnesses. So it is best to make sure the rabbit doesn't have any other signs before you judge what is has. First of all, diarreha is often caused from bad feed. I have found mineral oil in small doses, wipes the diarreha out really fast. I also feed rabbits hay and oatmeal when they have it, to try and give them more fiber because often they won't want to eat pellets.
Make sure the rabbit has plenty of water so they don't dehydrate.
Diarreha is very hard to clear up and if none of this works, go to a vet or ask for professional help, because if this stuff doesn't work, it is probably a much worse disease. Such as Enterotoxemia.

Enterotoxemia

has the following signs: profuse diarrhea, dehydration, reduced feed intake, and a rough coat. The rabbits die very quickly usually within 12-24 hours. The problem is usually an enlarged small intestine and the wall of the cecum is hemorrhaged. Bacteria is one of the causes of this disease, and it comes from poor sanitation. Also diets that are low in fiber and high in energy have this problem more. It is said that Enterotoxemia strikes a rabbity, disapears and strikes at a later date when you think it is gone. The most successful treatment is to change feeds, and keep the cages clean, but often it is too late.