You can absolutely still disconnect your item from your iCloud even if it is not powered on.
Please use a computer, and follow the below instructions for a successful removal of your item from your iCloud account:
- Go to the iCloud.com/find/ website on any server or browser on your computer.
- Click the All Devices tab at the top of the page.
- Select the device you want to REMOVE.
** DO NOT CLICK "DELETE DEVICE". THIS WILL NOT COMPLETE THE PROCESS. - Click Remove from Account.