function Get-Session
{
$Global:SI = Get-View ServiceInstance
$Global:SM = Get-View $SI.Content.SessionManager
Return $SM.SessionList
}

function Stop-Session
{
Process
{
ForEach ($Session in $_)
{
If ($Session.Key -ne $SM.CurrentSession.Key)
{
$Key = $session.Key
$SM.TerminateSession($Key)
Write-Host “Session $Key terminated.”
$Key = $null
}
Else
{
Write-Warning “Cannot terminate current session.”
}
}
}
}

The function Get-Session lists the currently active sessions. You can then filter them as you do any collection of objects in Powershell and pipe them to Stop-Session to have them terminated. Note that VI Client sessions appear to create two actual sessions and that the API (and therefore my function) will not allow you to terminate the current session.
Example:

Get-Session | Where { ((Get-Date) – $_.LastActiveTime).TotalHours -ge 8 } | Stop-Session

Enjoy!

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Here’s three ways you can make sure you don’t end up with unnecessary sessions:

1. Find scripts that lack the Disconnect-VIServer command:

$Path = “D:\Scripts”
$ConnectMatches = Get-ChildItem $Path -Recurse |
   Where {$_.PSIsContainer -eq $False} |
   Select-String “Connect-VIServer” |
   Where {$_.Line -notmatch “Disconnect”}
$DisconnectMatches = Get-ChildItem $Path -Recurse |
   Where {$_.PSIsContainer -eq $False} |
   Select-String “Disconnect-VIServer”
Compare-Object -ReferenceObject $ConnectMatches`
  -DifferenceObject $DisconnectMatches -Property Path

2. Disconnect old sessions through the VI Client GUI

On the Administration page of the VI Client, click on the Sessions tab. Sort by Status, Select the sessions that have been idle for a long time and click Terminate.

3. Use a script to disconnect old sessions

That’s more like it! Now who would want to go clicking and sorting about in the VI Client? ;) Thank goodness we have the VI Toolkit. Here’s the code:

$VCServerName = “YourVCServerName”
$HoursOld = 24        # Modify value at your pleasure

$VC = Connect-VIServer $VCServerName
$ServiceInstance = Get-View ServiceInstance
$SessionManager = Get-View $ServiceInstance.Content.SessionManager
$SessionManager.SessionList |
   Where {$_.LastActiveTime -lt (Get-Date).AddHours(-$HoursOld)} |
   % {$SessionManager.TerminateSession($_.Key)}
Disconnect-VIServer -Confirm:$False
Write-Host “The VI Toolkit ROCKS!”

Enjoy!

Related posts:

1. Remove vmware memory limits with Powershell
2. Calculate vSphere 5 Licenses with Powershell (UPDATED)