Free templates doesn't mean you have authorization to remove any copy-write requirements. There should be a TOS attached to the fair use of the template which should be presented as a requirement to downloading it or found within the download itself. It's also possible that there's indication within the files themselves specifying that you may not modify the files (which if the copywrite is located within the file you may not remove it). If it doesn't say you cannot remove it, than you can remove it by simply searching for it. Above is a grep command to find the files which will give you an idea of where to look if its in the filesystem. If its in the DB, you can run a similar search by selecting a LIKE on the text you're looking for (takes knowledge of the system structure to query though).
header('HTTP/1.1 420 Enhance Your Calm');