Yes! You can turn off the option to send your attendees the waivers or forms. To do this, select Waivers & Forms from your Extensions list and then click on Event Settings.
Locate Enable extension for this event, then move the green button to the "Off" position. This will prevent any waivers or forms from being sent to attendees that register for your event.
Once you are ready to send the waivers, simply move the button to the "On" position and Waivers & Forms will retroactively send the documents out for your new attendees' signatures.