If you are using the download version and are unable to unmark that box on the IRA Contribution Worksheet, it means that it is coming from somewhere else in TurboTax. Entries in box 12 or 13 of a W-2 or entries on the Keogh, SEP and SIMPLE Contribution Worksheet are the only places I know of that can do this.
With the download version of TurboTax, I suggest making a test copy of your tax file and deleting various items until the marking of the offending box. Start by deleting the IRA Contribution Worksheet and reentering your traditional IRA contribution just to confirm that the marking returns automatically. (You could do the same in the online version of TurboTax, but you would be working on your real tax return which is a bit risky and you would have to redo previous entries to restore items that you deleted.)
