You cannot edit quick create ribbon in CRM 2013 like you can do with form, home and sub-grid ribbon. You can neither disable it in supported manner nor can you remove Activities from it. You can not even add OOB entities which don’t support Refresh UI.
You can remove other entities from it like case, competitor etc…
To remove entities open the solution, Navigate to entity and deselect “Allow Quick Create”
Likewise you can add custom entities or refresh ui supporting entiries to quick create.