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You are free to remove any mention of XOOPS from the pages of your website and you can of course add your own copyright notice to protect the site design, e.g. copyright 'abc web design'.
The general rule with open-source CMS applications is that you should not state that the site is 'powered' by your own content management system, so 'powered by abc web design' would be considered ungracious, AFAIK.
I personally leave module developers credits intact in the admin side and remove them from the public side.
A thread is for life. Not just for Christmas.