In addition to the citrus idea, cats don't like sharing litter boxes, so
other urine will discourage them, too. (Once they stop coming, you can
stop.) They also like privacy, so if you can change things a bit to take
away any privacy, that will also discourage them.
A motion activated sprinkler can get your message across quickly, too. And
if there's something spikey, they won't walk there. Trimmed blackberry or
rose canes are not fun to walk on.
You may need to try a combination of these things, and it won't really be
any of them that'll do the trick, but rather the ongoing bother facing the
cat. They'll eventually find someplace nicer for them.
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