How to delete unused document type

I have deleted my Integration Component. Now the document type are left behind. When I try to remove these used document type, it is saying I cannot delete because some client group is using it. I also remove them from hte client group, but still cannot delete these doc type.

Now I know that I should have deleted the document types before I deleted my IC. Is there any tricks I can use to delete these unwanted document types after their IC are deleted?

Thank you.