I'm not sure what your set up is or how extensive the flea problem is. I doubt the rabbits are the only place the fleas are so you must treat the surrounding area, any other animals (dogs/cats), yard if the rabbits are outside, etc. What follows is my experience with fighting fleas but **not fleas on rabbits**. MaggieJ et al will need to screen these recommendations in terms of what is safe and what is dangerous for rabbits.
I gave up on Frontline, Comfortis, Capstar, Advantage, and all the other vet recommended chemicals several years ago. Tried them all, none worked. "All-Natural, Essential Oil based products" worked better but not enough. Pig-farm-strength prrythim also did not work (the fleas danced and laughed).
What did work:
1. Treated yard with Beneficial Nematodes.
2. Treated house (carpet, furniture, floors) with Borax -- sprinkle on, leave for a few hours or overnight, vacuum up. Repeat every few days for several weeks to get past the lifecycle of the flea.
3. Bathed the dogs every couple days in flea shampoo (with coconut oil to keep from hurting their skin) and flea combed them daily.
I did that two years ago. This year is the first time since that we've see a flea one. The monsoons have brought them back.