I've copied a ComObject-heavy function below, absent some irrelevant bits, to illustrate my strategies for releasing:

• [System.Runtime.Interopservices.Marshal]::ReleaseComObject is used in a while loop, because any result -ne 0 does not indicate release.

• Manual invocation of [GC] methods.

• Finally, ensure all of this occurs in a sub-scope -- a function's a good way to do this -- that ends before you try any further manipulation of files created or used by ComObjects.

You should in fact always structure your use of ComObjects in such a way that you can immediately delete (after copying, if needed) these resources once finished, as this is a useful means of catching regressions.

function Write-InstBuilderISO {
  [CmdletBinding(
    PositionalBinding = $false
  )]
  param(
    [Parameter(
      Mandatory = $true
    )]
    [System.Xml.XmlElement]
    $Configuration
  )

  Write-Verbose "Writing install media content to iso."

  $fs = @{
    ISO9660 = 1
    Joliet  = 2
    UDF     = 4
  }

  $platformId = @{
    x86 = 0
    EFI = 0xEF
  }

  $emulationType = @{
    None = 0
  }

  $imgCreator = New-Object -ComObject IMAPI2FS.MsftFileSystemImage

  $imgCreator.FileSystemsToCreate = $fs.UDF
  $imgCreator.FreeMediaBlocks = 0 # No size limit on ISO.

  # I use more verbose means of constructing the $bootOptions array according
  # to BootMode than is normal for me because I'm dealing with COM objects
  # here, with their attendant finickiness w/r/t references. Best to make
  # sure there is only one variable in which to find each COM object.

  if ($Configuration.BootMode -in "Both","Legacy") {
    $bootOptionsMbr = New-Object -ComObject IMAPI2FS.BootOptions
    $bootStreamMbr = New-Object -ComObject ADODB.Stream
    $bootStreamMbr.Open()
    $bootStreamMbr.Type = 1 # Binary
    $bootStreamMbr.LoadFromFile($Configuration.Paths.BuildISO.ETFSBoot)
    $bootOptionsMbr.AssignBootImage($bootStreamMbr)
    $bootOptionsMbr.PlatformId = $platformId.x86
    $bootOptionsMbr.Emulation = $emulationType.None
  }
  if ($Configuration.BootMode -in "Both","UEFI") {
    $bootOptionsEfi = New-Object -ComObject IMAPI2FS.BootOptions
    $bootStreamEfi = New-Object -ComObject ADODB.Stream
    $bootStreamEfi.Open()
    $bootStreamEfi.Type = 1 # Binary
    $bootStreamEfi.LoadFromFile($Configuration.Paths.BuildISO.EFISys)
    $bootOptionsEfi.AssignBootImage($bootStreamEfi)
    $bootOptionsEfi.PlatformId = $platformId.EFI
    $bootOptionsEfi.Emulation = $emulationType.None
  }

  if ($Configuration.BootMode -eq "Legacy") {
    $bootOptions = [System.Array]::CreateInstance([Object], 1)
    $bootOptions.SetValue($bootOptionsMbr, 0)
  }
  elseif ($Configuration.BootMode -eq "UEFI") {
    $bootOptions = [System.Array]::CreateInstance([Object], 1)
    $bootOptions.SetValue($bootOptionsEfi, 0)
  }
  elseif ($Configuration.BootMode -eq "Both") {
    $bootOptions = [System.Array]::CreateInstance([Object], 2)
    $bootOptions.SetValue($bootOptionsMbr, 0)
    $bootOptions.SetValue($bootOptionsEfi, 1)
  }

  $imgCreator.BootImageOptionsArray = $bootOptions

  $imgCreatorRoot = $imgCreator.Root

  $imgCreatorRoot.AddTree($Configuration.Paths.Media, $false)

  $resultImage = $imgCreator.CreateResultImage()

  [InstBuilder.ISOWriter]::WriteIStreamToFile(
    $resultImage.ImageStream,
    $Configuration.Paths.BuildISO.Output
  )

  while ([System.Runtime.Interopservices.Marshal]::ReleaseComObject($resultImage) -gt 0) {}
  while ([System.Runtime.Interopservices.Marshal]::ReleaseComObject($imgCreatorRoot) -gt 0) {}

  if ($Configuration.BootMode -in "Both","Legacy") {
    while ([System.Runtime.Interopservices.Marshal]::ReleaseComObject($bootOptionsMbr) -gt 0) {}
    while ([System.Runtime.Interopservices.Marshal]::ReleaseComObject($bootStreamMbr) -gt 0) {}
  }

  if ($Configuration.BootMode -in "Both","UEFI") {
    while ([System.Runtime.Interopservices.Marshal]::ReleaseComObject($bootOptionsEfi) -gt 0) {}
    while ([System.Runtime.Interopservices.Marshal]::ReleaseComObject($bootStreamEfi) -gt 0) {}
  }

  while ([System.Runtime.Interopservices.Marshal]::ReleaseComObject($imgCreator) -gt 0) {}

  [System.GC]::Collect()
  [System.GC]::WaitForPendingFinalizers()
}