Keeping them occupied will prevent them from eating non-food items when they are bored. They need stimulation exercises and things they can fidget with. You can train your dog by making them wear a basket muzzle whenever they chew on their collar (remember never leave them unattended while they're wearing a muzzle). PICA caused by medical conditions is more complex and you may want to consult your veterinarian. They will be able to prescribe a treatment thats tailored for the medical condition.
In addition you must further eliminate the possibility of your dog eating its Seresto collar. When putting it on make sure it is snugly fitted. Trim the excess length to prevent your dog from biting. Make sure you put it away where your dog cannot get to it when it is not in use.