Prometheus has brought to my attention the fact the two of you recently cornered him, then set about harassing, bullying and generally using your combined weight to intimidate him, eventually forcing him to give up his chosen name.
The suffix you took such exception to is - Firesong Thief. Clearly Prometheus intended no slight to your profession or guild, indeed he was barely aware of your existence at the time he took on the name. Had he chosen something like Thiefbane, I would tell him that your attentions were down to his own stupidity. In this case though, it appears that the two of you have happened upon the most measly of excuses to badger, bully and menace a good natured lad, who's barely off the little who list.
Prometheus is intelligent(ish) and reasonable. I'm sure that if you had politely asked him to choose a different name or if you had offered him something in return he would have happily changed his name. You will find though that the methods you choose to persuade him have had a counter productive effect. Bards have now changed their names in silent support of Prometheus, as have others who have heard about the nasty way in which you set about trying to exert influence.
Being of a magnanimous nature, Prometheus has agreed to strike a deal with you. If you successfully come to an agreement with him, one where he actually gets something in return, you will soon find this blip of an affair forgotten.
Lucky for you Prometheus does not covert material belongings. Barter away.
Written by my hand on the 9th of Hindyear, in the year 1209.