An aerospace engineer was slapped with a 16-month prison term for falsely claiming exempt status on his W-4s and $0 income on his 1040s for a dozen years. (U.S. v. Giambalvo, 8th Cir.)
If you see tax-protesting employees file false W-4s, withhold using single/zero status until they provide you with new valid forms. Consider a policy that allows for terminating employees who file false W-4s. Read about the case and lessons learned at www.theHRSpecialist.com/taxprotest.