BF, the thing is, you should WANT your bitch to fetch rabbits back alive, thats the mark of a good lurcher...!!
Remember, with edible quarry a lurcher is a 'catcher', not a 'dispatcher', thats YOUR job...!
Personally, you should encourage her to do so and take them off her in a calm and praising manner.
She's young and, by the sounds of it, level headed, she knows from that first pick up that they'll offer her no problems and return 'em to you, unharmed.
Now.. It'll be a different story when she tackles sommat a bit bigger that will bite back... And whether she kills her bunnys no