Only when they file their taxes (including for a refund). And alternately, they could cease withholding altogether and it would be up to the employee to either set it aside themselves to pay or send directly.
And when since is anything a "crime" anymore when this current administration has continued to VIOLATE multiple laws, including the Antideficiency Act, the various Civil Service Acts, the Impound Act, and are in essence doing "ex post facto" punishments-through-E.O.s. 
What is your response about this current administration's VIOLATION OF NUMEROUS LAWS?
( *crickets* ) 
They are still working out the details of how to go about something like this but if you'll notice, the GOP - and most notably, the state of TX, comes up with similar "policy schemes" to benefit THEIR STATES continually, and in some cases, have actually prevailed with them.
We need to start going on the "STATES RIGHTS" offensive since we obviously no longer have the federal government (nor the SCOTUS) to back us up this time.
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