Log into your CPanel page > go to Account Level Filtering.
Filter name: write the e-mail address you want to ban.
Rules
Select From. Select “matches regex”.
Below “From”, write the e-mail address that you want to ban, for example: “test@gmail.com”.
Actions: Discard Message.
Save.
That is pretty much it.
If you select “From” and “Equals”, it won’t ban the e-mails. You just need to select “matches regex”. It won’t ban other Gmail e-mails.