You can remove the Domains/sub-domains from the Plesk Panel. 

1. Login to the Plesk Panel.
    
    
   
   
2. Go to the Websites & Domains tab. You will get all the Hosting websites under your webspace.
   
   
3. Click on "Show More" to expand the further options. 
    
    
   
   
4. Click on "Remove website" to remove the domain. 
   
     
   
   
5. In the confirmation window, Click "OK" to remove the website including all the data. 
   
    
   
   

Note:

• If you remove a domain, it will also remove the subdomains, email accounts, Databases, Web-contents etc.
• If you remove a website (domain/subdomain), it will delete all related files, directories, and web applications from the server. So be careful while performing the remove task!
  
  

→ Looking to add domain to your Plesk? Please refer to add domain on Plesk for more details.