You've hit the nail on the head--with PKOs, the military contingents remain under national command authority, and the most the UN can do is ask that they be sent home (hopefully to face discipline, but often not).
With UN civilian personnel, there are at least administrative disciplinary measures that can be taken (up to and including firing them), although it seems very rare that this ever happens.
The issue has become particularly problematic with regard to issues of sexual abuse, as I'm sure you know--hence the adoption of a "zero tolerance" policy. With military peacekeeping personnel, however, still requires that troop contributing countries cooperate.
According to the UN:
I haven't seen more recent statistics--nor does it say what happened to the non-civilian personnel when they got home..Between January 2004 and November 2006, 319 peacekeeping personnel in all missions had been investigated, resulting in the summary dismissal of 18 civilians and the repatriation on disciplinary grounds of 17 police and 144 military personnel.
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