There are no preferences to disable the desktop notification in the Plays.tv client or in your Account Settings.
However, you can still disable them through Windows Notification Settings:
For Windows 7:
Right-click over the time and date display. Choose the option for 'Customize notification icons'.
In the Control panel that comes up, find the Plays.tv client on the list. In the drop-down menu next to the program, choose 'Hide icon and notifications'. This will disable the notifications.
For Windows 10:
Choose 'Settings' on the Windows menu. Click on the System Icon.
If you click on the Notifications & Actions item, you'll be presented with a list of programs. Merely turn off the Plays.tv client switch to disable the notifications.