You need to follow the below step to remove AD tenet from azure. if you are going to delete the abc.com from azure AD
1) Fist you need to delete the all users from azure portal for the abc.com
to remove bulk user you can use the below steps
Get-MsolUser –All | Export-CSV c:\users.csv
Edit your CSV and remove any accounts you do not want to delete (ie, your account and other Global Administrative accounts!)
Save the file.
So let’s now remove those users by using the following command:
Import-CSV c:\users.csv | Remove-MsolUser –Force.
2) stop you ad sync with azure portal
Connect-MsolService -Credential (Get-Credential) #provide your global admin username and pwd
Set-MsolDirSyncEnabled -EnableDirSync $false -Force
3) once you deleted the users from the ABC..COM & stop the sync then go to aad.poral.azure.com and navigate to azure active directory.
select the abc.com and click on the delete option.