Oooops shit I just realised you were asking for naturist Spas not naturist beaches, sorry.
Piscina Bernat Picornell organises Naturist days (Saturdays if I remember well), hidromassage, climatized pool, sauna etc and fantastic premises and swimming pool built for the Olympics in 92, and it was really cheap when I used it a couple of years ago. Is located in Monjuich near the Olympics Stadium.