The first step to rejuvenate old plastic planters is to disinfect the planters by soaking them in a tub filled with water and bleach. You can then use a plastic scrub to scrub away the rock. You should then rinse off the plastics using a hose so as to ensure that there is no bleach residue that may cause harm to the plants.
I think you mean weed barrier with stones on top, I think. Mind you it's pretty much impossible to tell from the actual wording of the question. So, if it's weed barrier underneath stone there is no reason to remove it. You can pull away stone, cut the barrier, dig a hole and plant something. Yeah.