Then shoo that cat away from your garden, feeding a cat potentially poisonous or toxic food is not only illegal (animal cruelty etc), its also a terrible thing to do!
Just shoo the cat away and it will get the message that it's not safe. There is no need to make the poor little thing sick. It's an animal and its hungry, it sees you as a human and potentially someone who it can rely on to feed it (if its own owners aren't doing it properly).
If it's a stray you can call the local animal rescue place and get them to come pick it up.
I'm with eyeheart, compost is a great solution. You might turn them under so they aren't a temptation, then if a critter (dog, cat, or other wise) digs 1 up and eats it, it'll probably just have the symptoms of bad take out. It's still not a diet I'd encourage to any of my animals.