Let’s see the steps to disable AD Sync, remove AAD connect and move to cloud only administration.
1. Download Azure Active Directory PowerShell Module from following location. Complete the installation.
2. Click to open the PowerShell using the shortcut created by installation in previous step.
3. Run the following command to connect to the Azure Active Directory of the tenant for which you are disabling AD Sync.
4. Enter the Global Admin credentials for Azure AD/Office 365 tenant.
5.Run the following command to disable the Directory Sync.
Set-MsolDirSyncEnabled –EnableDirSync $false
6. Run the following command to verify if Dir sync is disabled.
7. Go to your AAD connect server and uninstall Microsoft Azure AD Connect using Uninstall Program in Control panel.
8. Additional clean-up steps (Optional).
a. Remove service account created by AAD connect from AD. Check Users container for an account like MSOL_GUID. Description of the service account has the name of the AAD connect server for identification.
b. Remove the local AAD groups created by AAD installation, if you are not planning to install AAD connect again. If you have installed AAD connect on a DC, these will be AD group. Keep them for other AAD connect installations or if you have or plan to have.
c. Remove the directory structure.
d. Remove the DB if you were using a remote DB server.