How can I stop a cat from coming in my garden? (He is trying to get at my pet rabbits)?

Fence off the area around your hutches. Make sure it is high enough that the cat can not easily jump over it, if its tall enough it will discourage the cat from even trying. Four feet or so should do. Also, try planting some marigolds. They are used to keep raccoons and other pests away, but I found that our cats are not fond of them either. Good Luck!