If you suspect that the email you have just received is a spam, you can mark it as a junk email. In order to do this, click on Junk mail in the issue view.
Such e-mail will be then deleted, as well as automatically added as blacklist filter on mail server and mails from this e-mail will never be fetched again.
You can also move suspicious email addresses to blacklist, which is available in project Settings, Helpdesk tab. Emails coming from such emails will never be fetched.