I have the same problem on mine, for the second time. The first time was just after I bought the aerobars and the LBS fixed it by providing a new pin. They've lasted for several years now, and the pin just came loose again. Like you, I drove it back in with a punch but it worked loose again.

I don't think you can do anything with the old pin, so what I'm going to do is put in a small finishing nail, so the head won't go through, and then burr the other end enough so it won't go through either.

The first time it happened that's basically what the LBS did while they were waiting for a replacement pin.