Marcus, not knowing which model you have makes helping you a little more difficult. Check for a serial number on the back or base;
Identify your iMac model
The model/year will tell us what operating system the device can run and to a degree if it's worth the effort.
You may be able to boot into the recovery partition, turn on holding Command + R keys. If so then it may be possible to use Internet Recovery after erasing the HD.
What exactly do you see when you boot the device? Eg, Apple icon on black bacground/white background, progress bar, login screen with empty dialogue boxes. When does the "disabled" notification appear?
Not knowing if/how this iMac has been disabled my first thought would be to try booting it from a different bootable source such as a bootable USB macOS installer on a thumb drive. You can make one if you have other Macs or buy one on Amazon but first we need to know which macOS will work. If it can be booted this way it may be possible to erase the internal drive along with its lockdown but if it is some sort of firmware, or hardware lock you may be out of luck.
