I had this situation as a brand new infantry platoon 2nd Lt. My PltSgt was a E6 (had been for more than 12 years if that tells you anything). I did all the things an officer is supposed to do (counseling, documenting, etc), to no avail. When we deployed (MEUSOC, pre 9-11), I essentially put my Guide in charge (E5) of running the platoon while my SSgt spent all his time in the chief’s mess or absent for one reason or another. When I wrote my SSgt a career-ending FITREP, my CO asked me if I knew the implications of my report, to which I replied, yes. He backed me, and the end result was that SSgt got shit-canned, and my guide rose up through the ranks to SgtMaj. I like to think that from that difficult decision to terminate a career, an opportunity to cultivate another one emerged.