Try speaking bunny and make your hands a little less appetizing if that doesn’t work.
What I mean is this: a bunny lets out a high pitched squeal when it is in pain, so rather than saying a normal voiced Ow!, try squeaking or squealing (not too loud is fine) when he bites you.

If he is really young, he hasn’t learned to set his teeth but not hard. Many bunnies use their teeth like another hand to communicate what they want and know they are not biting to hurt and never break any skin. That’s the control he needs to learn. My Samantha does that all the time. Also, if he is in puberty now, he will bite when he gets too excited, and it’s getting to be time for a neuter.

If the squeal doesn’t work, you can put a light dab of vinegar and water on your hands to make yourself less tasty. If he licks first, he is kissing you and tasting the yummy salt. But I wouldn’t try this yet because you want him to continue to let you hold him and pet him, so you don’t want to make your hands something he dreads. Start squealing first.
BTW, corporal punishment, ie. hitting, bopping etc. does not work on rabbits. Many will learn to understand “NO!” with a bad behavior, but it means disapproval to them and they may do it again. You want him to think he’s hurting you. He would rather not.

Are you restraining him when you hold him? That might be another reason he is nipping, to express his displeasure with that. If he doesn’t like being held, let him come up to you for pets on his own terms.

Yeah, squealing at Leopold sure as heck doesn’t work. When he bites, he means business. I’ve been left black and blue by him many, many times. Once he even broke the skin. I’ve just learned to wear a really heavy sweater when I have to pick him up.

Rabbits don’t generally like to be held at all. They will tolerate it when they are very young, but as they get older, they will not put up with it (most buns, there are exceptions, but that’s rare). Interact with her on the floor so she has the option to come up to you as she pleases. If she will allow it, you can try to gently stroke her forehead. Forcing holding will not do anything to help your relationship and this could strain her trust in you later on. Has she been spayed yet or is that coming up?

Female rabbits have an extremely high rate of reproductive cancers by the age of 3 (something like 80%). Getting them spayed is definitely necessary, even for an only bun. Females also tend to be territorial when not spayed and aren’t very fun to live with.

In the wild, anything holding a rabbit is usually planning to eat it for dinner; once you understand this, it’s easy to see why most rabbits instinctually do not like being held. Baby rabbits probably tolerate this better because they are so small and helpless, but I have known very few adult bunnies that would let people hold them without putting up a fight. People seem to have a misconception that rabbits are creatures that can be picked up and snuggled, but they’re really not! It doesn’t make them any less fun though .

Regarding the spaying, like Elrohwen said, it’s not only to make sure that she does not have babies.  Besides the tremendous health benefits, rabbits can become aggressive, territorial, and more destructive once they hit sexual maturity and the hormones start flowing…spaying can help stop these kinds of behaviors.

Getting her spayed helps 100%. It prevents false pregnanies, helps with litter habits, and the chance of reproductive cancer goes away.

Hormonal rabbits hate being held, as well as fixed ones.. holding is a nono to them, but it is needed when it comes time to clip their nails or to be examined by the vet.

Also, many baby bunnies are ok with being held because they’re small and scared. We held Otto all the time when he was 8-10 weeks old, and after that he became a terror. Only after a couple years of working with him can we easily pick him up again, but he certainly doesn’t enjoy it and we get foot flicks once we put him down.