Go to Admin > Workflows > Automations > Ticket creation > New rule and set up a rule as shown below:


Conditions:

In Tickets > If Requester email > Contains any of > test.com OR spam.com


Actions:

Mark as spam



You can also reorder the rules added. Here's a video that can help - https://www.youtube.com/watch?v=Gt2VWBeuBU8