Purging WDS database and clearing prestaging values from AD computers

Purging Auto-Add Database in WDS WDS purges approved computers from Auto-Add database every 30 days by default.
You can also change the retention period for approved computers record by running WDSUTIL /Set-Server /AutoAddPolicy /RetentionPeriod /Approved:Days command. (http://technet.microsoft.com/en-us/library/cc754289(WS.10).aspx)

To change the length of time approved computers are held in the Auto-Add database to 7 days.
WDSUTIL /Set-Server /AutoAddPolicy /RetentionPeriod /Approved:7

If you wan't to manually purge approved computers in Auto-Add database you can run wdsutil /delete-AutoAddDevices /DeviceType:ApprovedDevices command, or to delete all (approved, pending, rejected) you can visit this URL (http://technet.microsoft.com/en-us/library/cc770832(WS.10).aspx).

But this process doesn't clear computer's RemoteInstall/NetBootGUID property from Active Directory, so you might need to clear this value in AD.
Clear prestaging data in AD
If you also need to clear RemoteInstall GUID property from all prestaged machines in AD as WDSUtil only clears it's own Auto-Add database, you can use powershell commands below to do that.

• To see a computer's NetbootGUID
  Get-ADComputer -Identity ComputerName -Properties NetbootGuid
• To clear a computer's NetbootGUID
  Set-ADComputer -Identity ComputerName -clear NetbootGUID
• To list all computers have NetboodGUID value
  Get-ADComputer -Filter {NetbootGUID -like "*"} -Properties NetbootGUID
• To list all computers have NetboodGUID value by formatted output
  Get-ADComputer -Filter {NetbootGUID -like "*"} -Properties NetbootGUID,created | Format-List -Property name,distinguishedName,created,NetbootGUID
• To list all computers older than a week and have NetboodGUID value by formatted output
  Get-ADComputer -Filter {NetbootGUID -like "*"} -Properties NetbootGUID,Created | ? {$_.Created -le ((get-date).addDays(-7))} | Format-List -Property name,distinguishedName,created,NetbootGUID
• To clear NetbootGUID from all computers older than a week and have NetbootGUID value
  Get-ADComputer -Filter {NetbootGUID -like "*"} -Properties name,NetbootGUID,Created | ? {$_.Created -le ((get-date).addDays(-7))} | Set-ADComputer -clear NetbootGUID
• To clear NetbootGUID from all computers older than a week and have NetbootGUID value (Shorter : we only need Created property for date equation)
  Get-ADComputer -Filter {NetbootGUID -like "*"} -Properties Created | ? {$_.Created -le ((get-date).addDays(-7))} | Set-ADComputer -clear NetbootGUID

To get more detail about
Set-ADComputer : http://technet.microsoft.com/en-us/library/ee617263.aspx
Get-ADComputer : http://technet.microsoft.com/en-us/library/ee617192.aspx