The simple way to remove access to Themes is to remove them from SPTHEMES.XML file. However, please follow standard operating procedure and make as copy of the original file. You can find the SPTHEMES.XML in the C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS\1033 folder assuming you have a default installation. I have included an example below of the reduced SPTHEMES.XML file, after making these changes you will need to perform an IISReset to read the SPTHEMES.XML file back into memory with the changes you made.

SPTHEMES.XML

<?xml version="1.0" encoding="utf-8"?>

<!-- _lcid="1033" _version="12.0.4518" _dal="1" -->

<!-- _LocalBinding -->

<SPThemes xmlns="http://tempuri.org/SPThemes.xsd">

    <Templates>

        <TemplateID>none</TemplateID>

        <DisplayName>Default Theme</DisplayName>

        <Description>The default theme has a white background with blue control areas and orange highlights.</Description>

        <Thumbnail>images/thnone.gif</Thumbnail>

        <Preview>images/thnone.gif</Preview>

    </Templates>

</SPThemes>

Themes are a good starting point on the flexibility of customizing your SharePoint Portal for a more definitive look at branding be sure to check out Heather Solomon's Blog @ http://www.heathersolomon.com/blog

-Ivan