If you had asked to cancel all the workflows associated to a SharePoint List, you don't need to go to UI interface and do Remove Workflows .Here are the list of power shell commands given below.
$web = Get-SPWeb "http://mysite/testsite";
$web.AllowUnsafeUpdates = $true;
#List Name
$list = $web.Lists["Customers"];
# Iterate through all Items in List and all Workflows on Items.
foreach ($item in $list.Items) {
foreach ($wf in $item.Workflows) {
#Cancel Workflows
[Microsoft.SharePoint.Workflow.SPWorkflowManager]::CancelWorkflow($wf);
}
}
$web.Dispose();
*Customers is the list Name where are removing all the workflows associated to it.