Do you have a column for Misc Expenses or Petty Cash?

Paying tax on a bonus sucks, but they still benefit from the remaining amount- it still adds to their FICA contribution, although it is possible to have each employee fill out a W-4 form, to change the amount of tax on one pay check, up to four times each year. If they want to pay nothing, the usual amount or one penny, it's up to them.

You could also work with them to adjust the amount of tax withheld on their usual checks, too. The ones who always receive a large refund and constantly complain about never having enough money during the year would benefit from increasing the number of dependents on their W-4. If they prefer to keep the same number, it's their choice. As an employee, I always liked having the money during the year and paying or receiving a small amount at tax time.