New-TenantAllowBlockListItems -ListType Sender -Block -Entries emailaddress
Here is a super useful one liner that has come in handy more recently.
What this does is adds a user or domain to the tenant block list as it’s written below. If you need to add someone to the allow list, you can change -Block to -Allow. You can even change set a duration for both Blocking and Allowing.
Example with Parameters: