I found an interesting artlicle with a C# Snippet--enough to be intriguing but not enough for me to code from: 2013 - cleanup tasks from workflows and deleted items - SharePoint Stack Exchange :
int id = Convert.ToInt32(taskItem[SPBuiltInFieldId.WorkflowItemId]);
SPListItem item = list.GetItemById(id);
string workflowInstanceId = taskItem[SPBuiltInFieldId.WorkflowInstanceID].ToString();
SPWorkflow workflow = item.Workflows[new Guid(workflowInstanceId)];
If (workflow.StatusValue = SPWorkflowStatus.StoppedByUser)
Looking at SharePoint Manager, it seems that the Workflow Tasks have a field collection which contains among other things "WorkflowInstance". I'd like to achieve this functionality in PowerShell to clean my Workflow Task.
I'm also looking for a way to cancel Workflows of deleted Items.