From the Desktop (where you land after signing in), click once
on Mailbox to highlight it.
Go to "Collaborate" on the menu bar and choose "Rules..."
Click on "New Receive Rule"
On the "If" dropdown menu, choose "the sender". Automatically
FirstClass will open one more dropdown menu and a text box.
Enter the users address in the textbox on the right.
On the "Then" dropdown menu, choose "reject" or "delete
silently", and click save to close.