Did you already create the conference? Do you need to delete some of the Time slots now? You can do that by following these steps: 

1. Click on the Class name in the left navigation tray
2. Click on Signups in the Updates tab
3. Navigate to the Event Details screen of the Conference
4. Scroll down to the slots
5. Click on the Options button to the right of the time slot which needs to be deleted
6. Click Delete Slot
7. Slot Removed Successfully dialogue will be seen