Block mails from fake accounts

The docs should be helpful to you here I think:
Default Address - cPanel Documentation

 

Are you talking about emails sent from scripts?
How does anyname @ yourdomain.com send email out of an account?

 

Hello,

Many times when you see emails being sent from non existent accounts, they are not actually being sent from your server. Have you actually confirmed that this is what is happening? I ask because most likely what is happening is that you're the victim of a Joe Job. This is where email is being sent out (not necessarily from your server but another compromised server) with forged from and reply-to headers of non-existent accounts at a domain on your server.

If you have the default address set up (as Infopro mentioned) to deliver those messages rather than bouncing them with "Fail no such user here", then you will get a ton of them and it looks like they are coming from your server when in fact they really are not.

Unless there's a script involved on the account in question, there is no way to send email from a non-existent account.