If you're receiving a large amount of mail with the subject rewritten to begin with ***SPAM***, then it may be time to look at enabling the spam box on your account. The spam box is a folder where all of the e-mail flagged as ***SPAM*** will be automatically sent, bypassing your Inbox completely.
To enable the spam box, start by logging in to cPanel.
Under 'Mail', select 'Apache SpamAssassin':
Under 'Spam Box', click the 'Enable Spam Box' button if the option is not already enabled:
The next time your account or domain receives an e-mail that the server considers to be spam, the "spam" folder will be created and these messages automatically moved to it. By default your mail client(s) may not see this new folder, and particularly if you are using IMAP, you may need to refresh or resubscribe to the spam folder server side in order to view its contents. For example, using the software 'Postbox', we needed to specifically subscribe to this spam folder in order to see it:
After ticking the box and subscribing to this folder, it was then visible within our mail client.
Important notes: The spam box is, by default, never automatically emptied by our servers. This is to provide you with the ability to search the spam folder for any e-mails that may have been inadvertantly flagged by mistake. It's a good idea to automatically empty this folder manually every once in a while to prevent your mailbox from running out of space.