If you have access to all devices that you have installed Studio One on, you can go to Studio One>Log Out with the program open. If you need to remove the credentials that are stored locally on the machine, please refer to the following article from our knowledge base.
http://support.presonus.com/entries/68870180-Studio-One-3-Invalid-Login-and-Email-Error-
*If you don't have access to the computers that the software is installed on, the best idea might be to change your password in your my.presonus.com account. That will log everything out and prevent them from logging back in.
