A disciplinarian is a person who administers discipline or punishment to another. In a spanking context, a disciplinarian would be the spanker. In a BDSM context, a disciplinarian might be either a top, or a dominant, or both, or perhaps even a master.
The term implies a person who administers discipline, or supervises self-adminstered discipline. The disciplinarian may also be the person who determines what discipline is needed and why, or may be acting at the request of another — perhaps even the recipient of the discipline (this is a form of "topping from the bottom").