1) Disable the footer content to be displayed
Remove/comment the following line in the main.vmd file which could also be accessed via Administration > Layouts > Main Layout (if you are using the "Default theme"):

#parse ("/decorators/includes/footer.vmd")

2) Modify the message rendered in the footer
This can be done in the ConfluenceActionSupport.properties file. See Modifying Confluence Interface Text for a step-by-step guide. The following line, is a mapping key to render the footer's message:

powered.by.atlassian.confluence

{htmlcomment}
http://support.atlassian.com/browse/CSP-12486
{htmlcomment}