Partially jokey answer: An automatic sprinkler and a motion detector
https://www.youtube.com/watch?v=ElcviGYMb3U
Serious answer: I don't think there's much... I used to have a problem with cats pissing on my bicycle (which used to be parked outside) and the only thing that would keep them away was a bike alarm - at the expense of mine and my neighbors' sleep. Covering the bike made no difference, they just pissed on the cover instead. The fix ended up being just parking my bike inside.
If you let the garden get a little bit overgrown, do they still come and drop their shit there?