How to deny users from disabling my C# service

[C# Programming]

Hi everyone, I have a problem, we borrow our clients laptops in our company for a short time. Users should be the local admin of the computer in order to add/remove programs they need. However, some of them didn't return the laptops in time so, we decided to write a program that locks the computer on date that we give before. I have one windows service, one form application which interacts with user and communicates with the service. While I testing, when I kill my program or my service, I give bsod but, when I try to disable the service and restart my computer, it doesn't run anymore. I need a method to prevent disable my service. Even my form application gets killed, my service has to run in background. Thanks.