If an agent blocks a customer, then all the tickets created by that customer or 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 sidebar, click on Customers.
Click on the customer name you want to block.
Click on the triple dot in the top right-hand corner.
Choose Block.
You will be shown a confirmation message.
Click on Block.
There is another way to block a customer. Whenever a ticket is marked as "spam," then a confirmation modal comes up asking if you also want to block the customer who created that ticket.