So if you modify the account on-prem and then do a delta sync with AADConnect, does it update it?
If this account licensed? Can you simply remove it from Azure if not and resync if the above does not work?
Remove-MsolUser -UserPrincipalName "davidchew@Company portal .com" -RemoveFromRecycleBin
Only do this if the account is not licensed or tied to anything in Azure!!!