Block a User from Signing In (Microsoft 365)

1. Go to the Microsoft 365 Admin Center:
   https://admin.microsoft.com
2. Navigate to Users → Active users
3. Search for and select the user you want to block
4. In the user panel, select the Account tab
5. Locate the option Block sign-in
6. Toggle the setting to Block this user from signing in
7. Click Save changes
8. Confirm that the user status reflects as blocked
9. (Optional) Verify that active sessions are revoked by resetting the user’s password or using sign-out options
10. (Optional) If further access is required (e.g., mailbox or OneDrive), assign delegates or permissions before or after blocking access

Reference:
Prevent user sign-in and block access to Microsoft 365 - Microsoft 365 admin | Microsoft Learn