Well I haven't actually been into the Thieves Guild, but all the thieves in Riften proper are douchebags and I want to kill them. However, my character isn't a murderer, so I was hoping that there would be some excuse to kill everyone.
Can I at least wipe out those black briar dicks or whatever they're called
I have already tried. In an attempt to remove that questline from my menu (i have no ambition to do it with my current character) I entered the sewers to kill all of the thieves. I killed all of the common thugs easy enough, but once I reached the named guild members, I would beat their health down, but they wouldnt die.
So unfortunately not. Lame cheapness prevents you from killing them completely.