You also need to prohibit access to MySQL database server from external addresses. To do this, in a firewall that protects your Panel-managed server, add or enable a rule that prohibits TCP and UDP connections to the port 3306 from all addresses except 127.0.0.1.
To use the firewall that comes with your Parallels Plesk Panel for Linux:
Log in to the Panel as administrator.
If you did not install the firewall component, install it:
Go to Home > Updates (in the Help & Support group).
Click the link corresponding to your version of the Panel.
Locate Plesk Firewall module, select the corresponding check box, and click Install.
Configure the firewall rule that blocks external MySQL connections and switch the firewall on:
Click the Settings link in the navigation pane.
Click Manage Firewall Rules, and then Edit Firewall Configuration.
Click the MySQL server link.
Select the Deny option and click OK.
Click Activate to apply the configuration, and then click Activate again to switch on the firewall.