Yes, this can be done by creating a workflow Automator from Admin-->Workflow Automator-->New ticket Automator. Set the event as “Ticket is created”. Drag and drop a condition block into the workflow and provide the condition “From email is” and enter the requester’s email address. Now add an action block and choose the action Mark as Spam.

Alternatively, you would also be able to delete the requester from Admin-->Requesters and further emails from the user would end up in Spam.