Impossible to delete this element because of existing dependent elements - by: mendo15

Hello,

I am trying to uncheck the 'Is Contact' checkbox from a resource. However I am getting this error:

Impossible to delete this element because of existing dependent elements - Activity (3)

I found the three projects where this resource was assigned as a contact and removed them, however, I am still getting this message.

Please advise.

Thank you in advance.