This discussion is interesting except no answer on this can claim to abort a network session that won't go away.
The only way I've been able to do this is manually with NSCONTROL KILLSESS=#Snnn. Commands that I've attempted to write myself have failed miserably, so I live with it. Besides, it happens only rarely and I haven't seen it happen in over a year. (Perhaps my parameters are finally correct in NMMGR.)