Although the setup of SSO might seem a little daunting, disabling it or turning it off is quite simple.
Got to Setup > System > Single-Sign On (SSO) > Edit Settings > Edit Configuration > Then scroll to the bottom, just above the Save button you will see the Enable box is checked. You will need to uncheck it and once unchecked Save it.
This will at that point disable SSO and your users will need to log on using Workfront credentials. Theses will be their email address (case sensitive) and the password (case sensitive) they setup originally.