If you have created too many FTP User accounts and some of these have not been used, and you want to remove an FTP Account, then you can accomplish this by following this tutorial.
1. Log in to your Plesk account.
2. In the Sidebar Menu, click Websites & Domains.
3. Below Domain Name, click on the appropriate domain name.
4. Under Files & Databases, click on the FTP Access.
5. Tick mark the appropriate FTP Account and then click Remove.