I just got my dorf black olive from the same bonsai nursery and my first impression is that if you cut the thorns off that would be like cutting off its arms.. the thorns are where new branches are formed looks like. I made the mistake of cutting off every single thorns on my Boganvilla tree and after closely watching it over the course of two years I found out that's were the flowers come from. I didn't get flowers until new thorns were formed a 8 months later. If you notice the zigzaging pattern of the branches then it looks as though the next branch is basically a elongated thorns. Anyway if you have a definitive answer by now I would love to get some advice from you about my newly acquired black olive
tree. Have a relaxing Friday afternoon.