I always use peppermint oil to keep squirrels out of my planters, there's always 1 or 2 planters each year that they take a liking to and dig in. Rabbits are supposed to dislike the peppermint smell too, although I've never had to use it to keep them away to swear to it. Sometimes I put a few drops on a cotton pad folded in half and held above the soil either with old bobby pins or plastic forks. If it's just flowers, you can put a few drops right on the soil, not sure if that's wise if you're growing vegetables though. Or something to clip the cotton pads around the edge of the planter.
But I do know it works to keeps wasps away from nesting in inconvenient places, squirrels, chipmunks, mice from nesting under the hood of cars, so I'm guessing it'll work to keep rabbits out too. It's certainly easy and cheap enough to give a try, just remember to redo it after rains.
