Can we require workers to take an unpaid lunch break?

Q.  We have several employees who work in the field at different locations. At the start of the day, they clock in at the office and then clock out when they come back at the end of the day. We've noticed a trend in employees writing in “no lunch” on their timecards. That's resulted in a huge increase in our OT wages. What’s more, the supervisors are approving the OT without verifying whether the employees are actually working through lunch. I realize that under FLSA we have to pay them, but going forward can we MAKE employees take their lunch break? – Chelsea, North Carolina