I've used bitter apple to stop the cats from chewing on my gardenia plant. I had bought a bottle of it years ago to try to get Disney to stop chewing her feet. It helps but you will need to reapply it often.

Rocky is my plastic chewer here. He will eat any plastic bag he can find. I'm so afraid he will get a blockage from it because no matter how well we think we've disposed of or put away plastic bags he still finds them. My sister was here this weekend and couldn't believe he would do that. She asked if it could be due to a vitamin deficiency.