Loaner Laptop Expiration software

I am looking for an Admin installed software that renders a laptop (or computer) unaccessible after a set period of time. The situation : We have laptops the we loan out to mangers for training new hires to use our in house developed software.
The problem arises when the managers don’t return the laptops in a timely manor. Then we are unable to provide to the next manager who asks for them. Some of them end up being used as the new hires laptop, which we have determined very insecure as these loner systems are not encrypted. I am looking for some options, to be able to recover these laptops after the loaning time is up.