How can one delete old workflow from the menu that appears above Otto as one right clicks on him, it is driving me crazy.

I am trying to delete the old workflows from the list that appears above Otto when you right click on him. I have tried all I can find on the internet (i.e. going to the source file and delete the workflow form there). While this work in deleting the actual workflow, a invalid link remain "above" Otto and the list is becoming monstruous and annoying.

Thank you

Patrice