Ask HN: Kill Switch for Your Application

recently i had to distribute an c++ software to a client, and was asked to implement a kill switch. i choose to use libSSH, handeling an authentication process at the start of main(). i opened a user with /sbin/nologin on a server and sent the software with an ssh public key to challenge the server with. the hostname and user are hardcoded in the code. what other options do i have, how would you do it.