We have a few cats in the street, and it appears that one of them is shitting in my gravel driveway (right next to the driver's door). How can I stop this happening? Is there anything I can put down to deter it? I though it was a one off, but when I went to get rid of the evidence (that I also stepped in!!) there were a few little parcels which makes me think it's been happening more often. Thanks in advance.