Long press the contact on the favorites page then select "set default action" from there you can set the action to be whatever you desire including a one press dial.
Delete the person from your gmail or phone contact first. Then go into whatsapp contact list, go into the menu and select refresh. this should get rid of the person.
I think you have to go into the Whatsapp and do it from there. However the Whatsapp contact list is built from the contacts profile so in some cases you would have to remove the contact entirely from your whatsapp contact list.
Don't know if this will do what you want, but you should check it out.
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