So you want to know how to change or remove Yootheme copyright link at footer in Joomla 1.5 Template? So did I.
I have tried to search each file like I did in my other blog post when I tried to find how to change or remove Youjoomla.com Copyright and designed by JoomlArt.com cases. Those two cases i solved by edited the php file related to each templates. In this case I feel like fooled (stupid me, but not dumb.. hehe). But don't worry my friends..
EUREKA!!! I Found it.

I want to tell you that this is Just Sharing All The Things I Know, before we go. And please read part of YOOtheme "Terms of use":

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2.4. Modifications

You are allowed to make any changes and modifications in the templates or products to suit your needs. It is not permitted to change or remove the copyright information in the source code. This includes the XML descriptor file and all PHP, JavaScript, HTML and CSS files distributed with our products. Of course, any visual copyrights, for example the copyrights in the template footer, can be removed.

However, you are not allowed to resale or redistribute these changes and modifications as GPL or other licenses. It is strictly prohibited to port YOOtheme templates and templates included in our extensions to other platforms and content management systems.
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Here We Go and This is It:
1. Login to Joomla Administrator Backend;
2. Go to Extensions -> Module Manager;
3. Search the Footer Module (with the Type: mod_custom, not mod_footer)
4. Just open it and look at "Custom Output", there you'll find the words "Validates to XHTML 1.0 and CSS 3" and "Copyright (c) 2009 YOOtheme";
5. Now it's up to you to... :=) .
Sorry for my bad english.. :=) .. and hope you can read between the lines.