Network admins everywhere cringe today as March Madness invades the workplace.  So what does an administrator do?  Well, if you have a commercial grade firewall you can block port 5001 access from all LAN computers to stop Slingbox.  You can also block ESPN360 access using ports 1765 - 1763.  Sling box is a bit more of a problem because it can be configured to use any port.  In that case, iterate through your network to find the slingplayer.exe process using Powershell 1.0.

$strCategory = "computer"
$objDomain = New-Object System.DirectoryServices.DirectoryEntry
$objSearcher = New-Object System.DirectoryServices.DirectorySearcher
$objSearcher.SearchRoot = $objDomain
$objSearcher.Filter = ("(objectCategory=$strCategory)")
$colProplist = "name"

foreach ($i in $colPropList){$objSearcher.PropertiesToLoad.Add($i)}
$colResults = $objSearcher.FindAll()

foreach ($objResult in $colResults)
{
$objComputer = $objResult.Properties;
Write-Output $objComputer
Get-WmiObject win32_process -ComputerName . | Select-Object CSName,Description,Processid,WS,Path | Where-Object { $_.Description -eq "slingplayer.exe" } | Sort-Object WS -Descending | Format-Table * -AutoSize
}

That script will iterate through all of the computers in your Active Directory letting you know which computers are slingboxing.  Change the process description Where-Object in the last for loop to look for other processes responsible for streaming video.  From there you can write a script to kill the processes or just connect to the computer itself using PSKill from Microsoft SysInternals.  You can also uninstall the slingplayer remotely if you have the capability to do it.

Schedule the powershell process to run periodically and repeat the process or automate the kill process until the user knows they're being watched.  March Madness can be madness on a small business network that doesn't have the resources to use big name firewalls and content filtering services.  Even without these easier to use tools at your disposal, there are ways to get your point across and stop the usage of the streaming video applications killing productivity. 

Unfortunately, if the Slingbox is being sent to a PDA then you need some sort of network filtering to stop the streaming video from this machine because Powershell doesn't help for PDAs and non-Windows machines.