How do I stop cats pooing on my garden?

Try grapefruit rather than lemon. The skin s much thicker so they last longer - if you like grapefruit, eat them and then use the peel, if not, quarter them, squeeze the juice over the area you want them to avoid and scatter the peel there. Remove any poo you can see, first. I found that grapefruit peel was more effective than actual gel sold for the purpose and also lasts better when it is rained on. Depending on how large the area is, covering it with cardboard and putting the grapefruit on top may help as it makes more of a barrier and mean it smells, looks and feels different from the area they have got used to as their 'safe place to poo' If you catching it in the act then shouting at it, a water pistol or squirter, or throwing something to land close enough to startle it may work (or throwing something soft at it) - cat's like to feel safe when they poo so if it no longer feels safe it will likely go elsewhere