How to handle insufficient earnings for deductions?

We have several employees who have requested set amounts be deducted for each paycheck for various things (e.g. $200/month for HSA deductions, $150 per paycheck additional withheld for taxes, and $500 per paycheck withheld for retirement contributions). Although not common, sometimes these employees don't work enough hours to pay their taxes and cover the deductions they requested. Typically, this accompanies illness, dealing with a death in the family, or some other issue that results in the employee already being hit financially, so when they receive a paycheck of $0 thanks to deductions, it makes them angry and adds to the stress they're already trying to deal with.

What are our options in these situations to prevent employees from having their entire paycheck eaten up by their optional deductions? We are considering creating a policy that states that if a deduction can't be fully met, the company will not make any elective deduction. This seems sound because, returning to an example given above, if we're authorized to withhold $500 and the employee only has $150 earnings left to withhold after taxes, withholding $0 is still a violation of that authorization, but so is withholding $150. If we're not going to withhold the full amount, it feels best to at least ensure that the employee sees a little bit of money in their check. Is that policy compliant?

If anyone can direct me to a good source on this, I would greatly appreciate it. Thanks!