If your cat is eating non-food items, such as rubber bands, towels, plastic, etc., it may result in life-threatening intestinal blockages. The eating of non-food items is called pica. The causes of pica are unknown but theories include attention-seeking behavior, attempt to obtain a nutrient lacking in the diet, or frustration and anxiety. Because these behaviors aren't well understood, stopping them might require assistance from a behavior professional.
Here are some things you can try:
Prevent access to the items your cat is eating
Change to a low-calorie or high-fiber diet to allow your cat to eat more food more often. Check with your veterinarian before changing your cat's diet.
If you think the cause might be anxiety, clicker training is a good behavior modification technique that mentally stimulates your cat.
If you think it is attention-seeking behavior, do not scold him or say his name your cat when you catch him ingesting an item. Instead try to startle him with a loud noise - be sure to try to avoid him knowing the loud noise came from you. Once he leaves the item alone, praise him and give him treats. Set aside 10-15 minutes 2 times a day so spend with your cat.
If you think it might be play behavior, be sure to keep a variety of toys on hand and be sure to play WITH your cat daily!