Powershell command to Turn off the session state service on the farm

Disable-SPSessionStateService [-AssignmentCollection <SPAssignmentCollection>] [-Confirm [<SwitchParameter>]] [-WhatIf [<SwitchParameter>]]

Detailed Description
The Disable-SPSessionStateService cmdlet turns off the session state service. If your farm does not depend on session state service, we recommend that you disable it.

Note: 
When session state is disabled, some Microsoft SharePoint Server 2010 services, components, and third-party components no longer function. Changing this property modifies the Web.config file on every content Web application on every server in the farm.