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.
Blocking a customer
- Click on Customers. 
- Click on the customer name you want to block. 
- Click on the three-dot menu. 
- Click on Block. 
- A modal should be displayed. 
- 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.