This appears to work well. It uses the olde ‘rwinsta’ command to work around some Powershell oddities.
# Get list of disconnected RDP sessions
$RDPDiscSessions = Get-RDUserSession | Where-Object SessionState -eq STATE_DISCONNECTED
# Disconnect each of them one by one
foreach ($row in $RDPDiscSessions) {
'Logging off ' + $row.SessionID
rwinsta $row.SessionID
}