If you want to enable the various spam filtering options in Plesk please have a look at this article on our blog:
How to setup spam filtering in Plesk
As always, should you require more assistance in relation to this issue, please contact our Helpdesk explaining your issue - and what you have done to this point, and we will be happy to assist you further.
Please see our full contact information, on our main website: Contact Us.
Alternatively, email firstname.lastname@example.org for assistance.