Release the buttons when the device vibrates or makes a "drop" sound.
For advanced users only (voids warranty, security risks):
If you have forgotten the PIN or pattern, you can perform a factory reset using the device's physical buttons to enter Recovery Mode. Turn off the device completely.
It sounds like you’re asking about — likely a smart POS terminal or handheld Android device.
The most reliable method is for the previous owner to delete the device from their MobiControl web-console , which automatically prompts the device agent to unenroll.