Squirrels are classified as rodents and have well-developed jaw muscles and chisel-like front teeth that can chew through just about anything. These front teeth are kept sharp by gnawing on hard objects such as woody plant materials, seeds, and nuts. Since rodents' front teeth grow constantly, they must be filed down, or they will grow to the point where the animal cannot eat, and starvation will result.
You could try using taste repellents like capsaicin (a hot pepper spray) on the plastic items. There are sticky substances registered for use with squirrels, but they are messy to use.
I put wire mesh over my feeder lid/top were they chewed on the top edge. The cats catch the ones around the yard. I am not a cat person but from growing up on a farm, they are useful for rodents!!!
I have a squirrel cookout myself!!!