OK....I've become a bit of an expert on this over the last few months with a problem of local cats (including our own!) using the garden as a toilet. Here is what I hve tried, some may be suitable for you:
1. Secret Roar (Lion Sh*t in a fertilizer) spread over the area.
I think our cats (female) fancied a bigger boy as the fertilizer attracted them rather than scared them off
2. Sonic Cat repeller (purchased from ebay/amazon)
This uses a proximity sensor that sends a high level noise out to scare them off. Seemed to have partial succeess but one of our cats goes right up to it, maybe she is partially deaf!
3. Water Cat repeller (purchased from ebay/amazon)
Similar above except plugs into the hose and then sets off a arc of water for a few seconds when it detects movement. Works a treat and we have noticed less cats stray into the garden after using for a month
4. Lemon Zest etc
No effect
5. Pepper
Very funny - but don't tell the RSPCA. We have a tortoise though so not good as a permanent solution. It did stop the cats returning to the same spot though
The cheapest option? Shifts with a water gun, after a few weeks they will get the idea
Hope this helps
