Doesn't work for me. I set the preference, remove the Apple Mail To Do label from GMail, and still it comes back eventually. I've also tried removing the ToDosMailboxName key and value from the Mail plist file (after quitting Mail, of course), but no combination of these steps keeps Mail from recreating the label in GMail—and I don't even use To Dos!

I've just tried this in Gmail, and it seems to have worked so far. Here are the steps I used:

1. Followed the steps above.
2. Quit Mail.app.
3. Opened Gmail in the browser.
4. Went to "Settings", then "Labels".
5. Removed the label there.

The next time I opened Mail.app, the disconnected triangle appeared next to the Gmail To Do, and then it and the Gmail To Do section disappeared. It hasn't reappeared yet.