Real simple:
start /B /W wmic.exe /interactive:off ComputerSystem Where "Name='%computername%'" Call UnJoinDomainOrWorkgroup FUnjoinOptions=0
You can either do an Enter-PSSession or use psexec.exe to execute. Put a reboot command in a batch and call it a day.
Make sure to create a new glassbreak account local to the PC, so that every machine has a specific account someone can perform work with.