If you see a cat urinating in your front yard, it is probably because that area has been claimed as his territory. Male cats spray urine around their territory to tell other cats that this is his place: "Back off if you are a male cat", or "Come and get me if you are a female cat". Unfortunately, all it tells us is that there is a nasty smell around. If it was your cat making the smell, it would be a good idea to desex him, but since he is not yours, the only alternative would be to deter him.
There are many things which people try to use to repel cats, but remember to be persistent and not give up too soon. Simple chemical deterrents such as citrus peelings or coffee grounds spread around the area where the cat urinates may stop him coming around. Some people suggest sprinkling pepper around, but if this gets into the cats eyes, it can cause severe damage, so this should be avoided. You can also buy sprays from pet supply stores, these usually contain scents which the cat does not like. Sometimes these can have mixed results but be persistent and if one doesn't work, try a different product. A couple of larger modifications you could make to your garden involve:
- cats prefer to dig in loose dirt so if possible, try placing some pebbles in garden beds
- cats also don't like the feel of pine cones so you might like to spread some pine cones around the garden beds
- planting prickly shrubs in the areas where you think the cat is getting in
- putting prickly cuttings around
- modifying your fences by making them slope outwards so the cat can't climb in
- attaching a thin wire about 5cm above the fence along its entire length, so the cat can't balance on it
Start with the more simple suggestions, and persevere. If you know who owns the cat, you could try contacting them and politely asking if there is any way for them to keep the cat at home.