RSO418: Object still used in DTPxxx

I want to delete a DSO. I already deleted the transformation rules that used to fill the DSO and so the DSO is now a solitairy, unconnected object. Still I get the message that the DSO is used in a DTP process.

There was a note in support stack 6 that described exactly this problem. However, we are at support stack 16 so this note is not applicable.

What do I have to do to be able to throw away this old DSO?