Disconnect device from iCloud want to disconnect my iMac from the iCloud

Good evening if you already tried the common way of accessing iCloud from the device and going to settings and removing the device; or accessing the iCloud from ur iOS and going to all devices and removing it : as well as attempting to access the iCloud from your desktop iTunes and removing the device if all fails

then access the master Apple ID profile and follow these steps

Via the Apple ID website

1. Log in to your Apple ID account at appleid.apple.com.
2. Scroll down to the Devices section and click the device’s name.
3. Click Remove from Account and confirm

Go to system settings > Apple ID > iCloud. Turn off any settings that you wish.