This scripts helps deleting TargetTypes in SDL Web 8+ which has components already published. This script needs Powershell Core service module to run.

DeleteTargetTypes.ps1

[CmdletBinding()]
Param(
  [Parameter(Mandatory=$true)]
  [string]$TargetTypeTcmId = 'tcm:0-17-65538',	#BusinessProcessType 

	[switch]$DryRun = $false
)

Begin{
  try{
    Write-Host "Preparing"
    $modules = "Tridion-CoreService"
    $modules | foreach {Import-Module $_ -ErrorAction Stop}
    $client = Get-TridionCoreServiceClient
  }
  catch{
    Write-Host ("Process stopped with error {0}"-f $_ ) -ForegroundColor Red -BackgroundColor Black
    exit
  }
}	
Process{
  try{  
    $publicationIds = (Get-TtmMapping).PublicationId
	  Write-Host ("Following will be updated:")
    foreach ($publicationId in $publicationIds){
      if ($DryRun){        
	    Write-Host ("Would mark UnPublish for Publication {0}  with Target Type ({1})" -f $publicationId, $TargetTypeTcmId)	
      } 
      else {	  
			$client.RemovePublishStates($publicationId, $TargetTypeTcmId)
			Write-Host ("Marked UnPublish for Publication {0}  with Target Type ({1})" -f $publicationId, $TargetTypeTcmId)
	  }
}
    Write-Host "Done"
  }
  catch{
    Write-Host ("#Process stopped with error {0}"-f $_ ) -ForegroundColor Red -BackgroundColor Black
	continue
  }  
}