If an agent blocks a customer, then all the tickets by that customer or the tickets created by any agent on behalf of that customer are marked as SPAM.
Here are the steps you need to take to block a customer.
From the left-hand side bar, click on "Customers".
Click on the customer name you want to block.
Click on the triple dot at the top right hand corner.
You will be shown a confirmation modal.
Click on "Block".
There is another way to block a customer. Whenever a ticket is marked as a "Spam" then a confirmation modal comes up asking if you also want to block the customer who created that ticket.