Hello Nhan,
Below are some answers to your exact questions:
1/ I wonder what is email address of Microsoft support service? I made a question ticket and get the reply from support@mail.support.microsoft.com, but It is reject my email when I reply it.
Check your Exchange message trace to see why it was rejected or check the headers. That is the official e-mail address from Microsoft Support.
2/ The hacker hacked my account and created 100,000 users (One hundred thousand users). How can I delete all of them?
Perhaps look into making a PowerShell script that deletes the user accounts:
Delete a user
https://learn.microsoft.com/en-us/graph/api/user-delete?view=graph-rest-1.0&tabs=http
You can perhaps put a CSV together and use a ForEach-Object loop to delete each $userId that you don't want.
Import-Module Microsoft.Graph.Users
Remove-MgUser -UserId $userId
If this is helpful please accept answer.