Based on suggestions from SPCOM 2012, EDAS now supports two new ways to restrict event registrations: First, chairs can set events to disallow multiple event registrations for one event and each attendee. (Indeed, this is the default, since it makes little sense for most conferences to have both a student and a regular registration for one event for the same person.)
Second, you can add a category name to the event. Attendees can only choose one event with the same category name. For example, if there are multiple parallel tutorials, you could give them all the same category name, and the attendee could not accidentally register for more than one of them.
Finally, attendees can add notes to registrations, e.g., to indicate special needs or arrival/departure dates.