Hi there! I stayed in Buba house in September last year in a twin room with a friend. We could definitely close and lock the door to our room and there was a small locker inside the room as well. I don't think that security is too bad at the hostel.
The rooms themselves aren't very nice as they are small, dark and cramped and were quite hot near the end summer. However, the location is excellent and given Barcelona's nightlife you probably won't be there much anyway so the room size shouldn't be an issue!
Ciao!