To alleviate security concerns it is recommended that you use a security measure, allowing to restrict access to control panel with administrator privileges from certain IP's. You can make use of this function by creating a list of IP addresses to which a restriction policy will be applied, two modes are available:
Notes on access restriction policies
If the second policy is used, it becomes impossible to remove all records from the list. When you attempt to remove the last record, the restriction policy mode is switched automatically to mode 1.
When you attempt to switch to the mode 2 with empty list, you are warned of impossibility of such action.
You will be informed if access from your IP address becomes unavailable due to your restriction policy misconfiguration.
To use the access restriction function, select the Server shortcut in the navigation pane. The Server administration page will open. Click the 
Access icon on the Server administration page. The Access restriction management page will open:
To add a network to the list:
Click the Add Network icon. The Network editing page will open:
Specify the network IP address and subnet mask, and then click OK.
To remove a network IP from the list, select a corresponding checkbox and click Remove Selected.
To edit a network ip or subnet mask, select the ip address in the list, and you will be taken to the editing page.
To set the policy mode, select the appropriate radio button and click Set. A confirmation box will open, prompting you to confirm the mode change. Click OK.
IMPORTANT
By default Plesk allows multiple simultaneous sessions for several users logged into the control panel using the same login and password combination. This feature might be useful when delegating the management functions to other users or in case if you accidentally close your browser without logging out, thus becoming unable to log in again until your sessions expires. Being the administrator you can choose to disable this capability.
To disable multiple sessions, follow these steps:
On the Server administration page click
Preferences.Deselect the Allow multiple sessions under administrator's login checkbox.
Click OK.