Officially, kiteboarding has been banned (forbidden) at the Costa Brava on (!!) December 24, 2001!!
Unofficially, you could do it during the low season (NOT July and August!!) at the beaches of St. Antonii, Pals and St. Pere Pescadore (approx. 80-100km north of Barcelona).
Be careful during the high season July and August. The cops might cause some trouble, especially when the beaches are packed with tourists!!!
http://www.kiteforum.com/phpBB/viewtopi ... 67&forum=1
to see the applicable postings here.