This is a bit technical, but it is a possible solution. You will need to create your own Flash file, but the basic instructions are here: The Forums at robrode.com - Controlling the Playback Component in Presenter
This would allow you to disable the "Next" and scrub bar on the playbar, you may also need to hid the Outline and Thumbs pane on the side bar. This way you could prevent people from moving forward in the presentation, while leaving the "Previous" button available to allow people to have backward navigation.
The only built in option that I know of that could kind of fit your need would be to lock the slides which would disable all but the play/pause functionality untill all the locked slides were viewed. So backward/forward navigation would only be available after the presentation was watched (or all the locked slides had been viewed).
From the Presenter Help Doc:
You can lock a slide for the specified slide duration. Locking can help ensure that viewers spend a minimum amount of time on a slide and do not quickly skip ahead. When you lock a slide, navigation controls are disabled along with navigation from the Outline and Thumbs panes. Quiz and question slides cannot be locked.
Note: Slides are locked only when viewed for the first time. Subsequent views of the slide are not locked and the navigation controls are not disabled.