My 28-month-old regularly loosens his car seat straps by pushing the button between his legs and using the weight of his shoulders to loosen the straps. I tried making the straps very tight, but he can still reach. I switched him to his older brother's car seat, but he will still reach. Most times I tell him "No button" and tell him to tighten the straps. He obeys, but it's not long before he's loosening them again. Inconsistently (and without much success), I tried taking away his blanket, toy or book, but that only resulted in a kicking tantrum on the back of my seat and immediate repeats. Fed up, a few days ago I started the weary practice of pulling over the car, spanking him, and firmly telling him "No button". It has stopped immediate repeats, but it is (so far) still a daily occurrence. How do I stop this behavior?