This bit of info is more than likely hardcoded, you will need to ftp into your site and locate the file itself and manually delete the section of code, depending on what format your theme is in, it may be located in the top level of your Concrete5 instillation in a folder called "themes" or it may be located in "packages", once you locate your theme, some themes will have a folder called elements or includes, in this folder you will find a file something along the lines of footer.php, once in this file you should be able to locate the code you are looking for.

If the theme you are using does not have an includes or elements folder then the footer may be attached to the actual pages, ie default.php, right_sidebar.php etc, in this case you will need to delete the line of code from every page.