What do you mean by user session? Is this the one you see under Planning Statistics?
If yes, Enable Use of the Application for Administrators should do it, but I would be extra careful before doing this.
If you are talking about the sessions that you see on EAS, then right click on the Essbase Server > Edit > Sessions > Click on the User > Select Kill or Log Off depending on what you are trying to accomplish?
This might help: http://docs.oracle.com/cd/E17236_01/epm.1112/hp_admin/ch10s02s05.html
The reason I said be extra careful is, you don't want to put the application into Administrator mode during a budget cycle. It will disable them to be able to connect the planning application.
Let me know if this is not clear.
Essabse uses the credential used during planning application datasource configuration on Essabse part for displaying sessions. Planning logistics tab can display the actual user name. I still find it hard to kill a sigle planning user session.