How do we prevent ‘stacking’ of FMLA, sick leave?

Q. We have an employee who wants to take a chunk of paid time off. She is eligible for FMLA leave. We want to start deducting the time off from her 12-week entitlement. Usually we require employees to provide a medical certification. But in this case, we are more interested in making sure she doesn’t take unpaid FMLA leave later after her paid leave bank is exhausted. Can we just do this?