You can choose which domain is served when visiting a specific IP addresses on your Server. To do this log in to Plesk and follow the steps below.
Click on the Settings option on the left hand menu then click on the IP Addresses icon.
On the IP addresses management page click on the IP address that you would like to set the default domain for.
Now in the default domain drop down box select the domain you want the IP address to display when viewed, then click Ok.
Now any requests to unidentified domains that resolve to that IP, or requests directly to the IP address itself will return the website on the domain you have just specified.