The easy way for that you would be to delete the requester from Admin-->Requesters and further emails from the user would end up in Spam.

But 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.