Currently a quick POC module against the undocumented Pulumi API (to help me clean up some stack mess I had made while getting ramped up on Pulumi).
Install-Module PulumiSH
Import-Module PulumiSH
# You need to have already logged in with pulumi.exe login
Connect-Pulumi -Org my-org
Get-PulumiProject
(Get-PulumiProject -Name my-defunct-project).Stacks | Remove-PulumiStack -Whatif