I've just taken a look at the Ryanair site. As far as I can tell, the 'free' flights actually have a 1p price tag on them, presumably so that their computerised booking system can charge a debit or credit card. (For instance, I found Stansted to Hamburg for 2p return, with no taxes and charges. However, you have to pay the usual 70p or �2 handling fee for debit card and credit card transactions).
As long as they don't mind paying 72p per time, there seems to be nothing preventing anyone from making multiple bookings. However, not all flights to any particular destination are included in the offer and many destinations don't appear at all. (For instance you can travel from Stansted to places like Esjberg and Eindhoven free of charge but the offer doesn't apply to flights to 'sunshine destinations' like Barcelona or Rome).
http://www.ryanair.com/site/notices/FREESEATSG RID070516.pdf
Chris