To the OP's original post...I think attacking your main character's 'fall' or dishonor works best if he/she is a captain. I don't know if other pirates or thieves would see the Federation as all that noble or have that much respect for Starfleet. Granted their insults could just be needling or a distracting ploy, but if this is going to be a running thing in your stories then I think your main character should really fall from a tall height.
That being said, like others have already recommended, really explore and explain-if to yourself before the rest of us-what would push a Starfleet captain to take such drastic actions. The time period matters a lot in that estimation, IMO. It seemed like the 22nd and 23rd centuries might have been more open to that kind of thing than the 24th, but that's more of my perception than something I can find on a show to substantiate it.
If it were me, I would probably go with a lower ranked officer. That way they can catch guilt and shame about losing their Starfleet posting and the missed potential. Plus I think it could be more fluid dealing with a lowered ranking officer. And even a Starfleet Lieutenant, for example, might have more experience or know-how than the average spacer.