Answer:
For adult rabbits, AdvantageTM (imidocloprid) is one of several safe treatments used to prevent and kill fleas, and is available from any rabbit-savvy veterinarian. However, you should not use such products on a rabbit before it is fully weaned, as it may be too toxic for a baby. Do not use any flea products on a rabbit unless a special "rabbit-savvy" vet has told you to.
You could also consider a flea comb for regular maintenance. It may take more time and patience but is non-toxic, and most rabbits learn to love the attention of being combed. So it has physical and psychological rewards for both you and your pet.