What if you don't have a refund for them to withhold?
Then they'll send the New Black Panther Party to muss ya up.
It will be handled like any other debt to the IRS, except the IRS agents who come to your door will be shirtless or dressed like Jill Kelley.
Then they will send you a statement of what you owe. If you still don't pay, they will send you a statement of what you owe plus penalties and interest. If you still don't pay, then after maybe a couple more rounds you will get a registered letter from a "special agent" of the IRS, which means criminal proceedings are imminent, and you had better get a lawyer. Expect that to cost one helluva lot more than the initial assessment, even if you don't end up having to go to prison.
See how it will work, now?