Saying NO only works when I'm home. They never eat them in front of me.

And I'm not getting cat grass because that will just confuse them - they could eat some plants but not others? Too confusing for them.

rub stuff on the plants

by raising your voice and saying no. You said cat so it will understand, it is harder with a kitten cause they are so full of mischief. My cat understands when I raise my voice, that is some place you do not go or something you do not do. When I whistle and call my cat it comes to me and sits in my lap. Just let your cat know what is right and what is wrong and believe me, it will understand. Even with a Christmas tree mine walks over and looks up at it and reaches up to touch a branch and I say No very loudly and she never goes back and does it again.

get gross tasting plants!
:]

Take Chili powder (the spice you cook with) and lightly dust or sprinkle your plants. This works great. Cats will not touch them again if the chili powder stays on. Make sure when watering you don’t rinse the chili all off.

put the plants higher above

Get them some plants that they can nibble on. Many plants are poisonous, so let me check, I'll be back.

I tried looking at the online Merck's Veterinary Manual, but if it discusses the subject index is not useful to find it.

You could try growing catnip, but that makes them go loco, and from experience the smell draws big mean Tom Cats around, looking for love.

Those Tom's cannot be reasoned with, and are under the impression that there is a harem of females around to fight for.

So, tell your cats about the dangers of catnip, before its too late.

get rid of one or the other....cats or plants... :-)