Humping isn't necessarily a bad thing, but the escalation into a fight isn't good. Are you able to pet Joann's forehead to calm her down a little first? If Howie can hump her for 10-20 seconds, he may be able to "get his point across" and then they can resume normal activities.
What kind of bonding space are they in, when are your sessions, and what do they have in the space with them? Sometimes adding or removing objects and litterboxes can help with territorial behavior.
I always had a very lightweight "cheapy" laundry basket in the room with us during bonding. When my pair would scuffle, I would use it to gently nudge them apart, then I would drop it upside down over one of the bunnies, making an immediate "cage". It was enough of a barrier to stop the fight, without ending the session. After a few minutes, the basket would come off and the session would resume. I never did less than a one hour session, and I never allowed a fight to end the session.
What other deterrents have you used, noise maker, spray bottle, and are you stressing them beforehand? Have you tried reducing the size of the bonding space?