How do I protect my band name so no one takes it from me?

We have a little problem in my band. The guy who plays the accordion called us all and asked us if we wanted to make a band so we all got together and it all worked out. Then the naming of the band came along, and the guy who plays the b guitar gave us our name.Now the accordion player is leaving and he wants to take the name with him. Im looking for someone to point me in the right direction to help the b player protect the name he gave us