If you have other code in “Custom footer” do not delete them because they may be used in other areas of your site.
This is best solution because:
User sees that they “agree to terms” and that they are responsible for ad that they post.
Will work even if you switch to other theme in most cases
No need to edit PHP files and be able to update theme to latest version without worries. (If you edit theme files, customization will be lost on next theme update. If you copy theme to other directory and rename it then it will not be updated in which case you will miss some cool features in future)