Brunobst
First step is to re-enable the account. this will allow you to act on the account so your changes will Sync.
2. Remove the account from Retention Policy, if one is applied.
3. Hide the address from GAL: Using Powershell: Set-Mailbox -Identity "ABC User" -HiddenFromAddressListsEnabled $true
4. When you remove the license for Exchange Online (Plan2), you must also remove the licenses for O365 Defender (Plan2)' and/or 'MyAnalytics (Plan2)', if presently assigned. These can be referenced programmatically as: EXCHANGE_S_ENTERPRISE, THREAT_INTELLIGENCE, MYANALYTICS_P2.
5. Disable the mailbox with: Disable-Mailbox -Identity mailto:email address removed for privacy reasons -Confirm:$false
Note: The '-Confirm:$false' phrase avoids the script pausing for your interaction, if running as an automation.
6. Then you can re-apply the Retention Policy, if it is still required.
7. Finally, you can disable the account.
Hope this helps.