My neighbour had the same problem, and it was with our cat! Lol..
We told her to keep a spray bottle by her back door and every time our cat came into her garden and scared her rabbit, she would spray her with water! It doesn't hurt the cat at all, but because they really don't like water our cat soon learned to leave the rabbit alone, all our neighbour had to do was pick up the bottle and she'd run.
Now, our cat leaves the rabbit alone! Lol..
Good Luck! I hope this helps!! Tell me how you get on!