Social security is not taxable if that is your only income.  It will be taxable if you have more than a certain amount of other income such as from a job, investments, or a pension.

Do you mean you have been having taxes withheld?  You can change that with the social security administration, and you can file a tax return to get a refund of the withholding.  (You won't be able to e-file unless you have at least a little income from interest or something.)

Or do you mean you have been filing tax returns and declaring the income as taxable?  I'm not sure how you would do that and not have the IRS reject it.  If you have been filing returns, you can file amended returns to get your taxes paid refunded back to you.