If it's mostly hanging off - I would remove it so it doesn't continue to dangle and bleed as he walks and the other spur hits it. The blood will end up on the backs of any hens he tries to mate, which could start something bad in the flock- chickens love blood.
If blood stop powder doesn't do the trick- and I just did something similar by the way, except it was the back toe, not the spur - his toe was hanging off, had no feeling and was causing an ongoing wound - so I used a scalpel (can be bought at a farm supply store) to take it off, which he didn't even feel. Unfortunately it kept on bleeding despite the powder, so I followed the excellent advice someone else had put on this site long ago (after washing all the powder off!!!)
Do this outside, the smell will linger. I learned the hard way.
Use an old butter knife you don't mind throwing away. With either a torch or the burner of your stove, heat the tip of the knife until it turns red- then cauterize the wound. If you're worried about hitting something other than the wound, wrap the surrounding area with a wet washcloth. Obviously have someone to hold him while you prepare/heat the butter knife- though I will tell you my guy didn't even flinch when cauterized.
The bleeding stopped immediately and he was back with the flock within hours. I did apply Vetricyn (also farm supply store) - then triple antibiotic ointment. I wrapped it with a non-stick pad and Vetwrap. Good as new.
) I hope it can help you too! 
