If you are using Azure Storage Sync and want to remove a on-premises server you may be tempted to just DELETE it from the the SYNC GROUP, but that will not do the trick.
If you want to deregister / unregister an on-prem server from Azure Storage Sync you have to:
- Go into STORAGE SYNC SERVICES
- Click on the Sync Service in question
- Click REGISTERED SERVERS
- Click on the server you want to remove
- Click UNREGISTER SERVER
- Type in the (copy / paste) the name of the server in the confirmation box at the bottom