Our company recently did away with comp hours. With this shift, FLSA exempt employees were told that if they worked under 40 hours a week, their PTO (paid time off) hours would be used to compensate. Can an employer expend these these hours without employee agreement in a salaried, exempt position?

Yes.

The only caveat is that once PTO hours are all used up, the employer cannot dock an exempt employee's pay in less than full-day increments and only them if the employee is absent for one of several specific reasons, such as the employee voluntarily taking a full day off for purely personal reasons.

FYI, the employer sets the hours of work, not the employees. Nor is an employer ever obligated to offer comp time.