Yes there are repellents that can be used....but not where food is concerned.
1- try putting a cage around the area in question
2- put a ltter box out side.
3-give your cat an area with soil where it can poop outside.
JUST SO YKNOW....although it may be gross for you...cat poop is good for soil.
As long as you wash your veggies...there shouldn't be a problem.