________ is defined as a scheme whereby tax on employees'income is dedcted at source by the employ?

The answer is "withholding".

The W-2 form is a contract authorizing the employer to withhold specified amounts. The employer is prohibited to dishonor the employee's withholding instructions. You can call the local IRS office and they will read thjose laws to you.

Times are rough, but citizens of the USA still have a right to due process of law, and the only process provided by law to seize money from a paycheck is the court order.