Open your Event Settings, and click on the RSVPs tab. Under "Accept RSVPs," you'll see the option to turn off the ability to allow guests to RSVP "Maybe."
The default event settings allow "Maybe" as an RSVP option.
If you turn off the ability for guests to RSVP "Maybe," guests will only be able to RSVP "Going" or "Can't Go."
If you turn off the ability for guests to RSVP "Maybe" after some of them have already RSVP'd "Maybe," their responses will stay as "Maybe," but if they re-RSVP they'll have to choose between "Going" and "Can't Go."