GreenJet diazo Theme for Plone has editable footer that is subdivided into two parts: footer with Plone info (1) and theme credits (2) with standard Plone colophon.
It can be customized as in default Plone: open Zope Management Interface and go to portal_view_customizations -> plone.footer. Just customize it according to your needs.
To hide theme credits (2) and Plone colophon you will need to add a special class to your ploneCustom.css.
#portal-colophon {
display: none;
}
Read our tutorial on how to customize CSS stylesheets in Plone.