I have created a Greasemonkey script for a website. What the script does is adding a div at the end of the page.
Greasemonkey now supports the
@noframes directive (long supported by Tampermonkey and Scriptish).
Use that for a cleaner way to block operation in iframes.
Unfortunately, npdoty's answer will now trigger a warning in Firefox's browser console:
Warning: use of return outside of functions is deprecated and may cause failures in future versions of Greasemonkey.