So, removing the device from your dashboard should have been the last step... However, let's see if we can fix this
Firstly, is the device in Apple's Device Enrollment program? If so, it will have to be removed here also. The easiest way to check this is to go to Systems Manager > ADE. Look for the serial number. Does it appear in the list? If so, you'll have to go to either school.apple.com / business.apple.com, find the device in the list of devices and remove it from ADE. A refresh of devices in Systems Manager > ADE should see the device disappear
Let me know if this resolves it.