How do I flag an extension which no longer works?
Is this an extension that is published in the CiviCRM extension directory? At the moment I do not think you can flag an extension as not working, but it would be nice if the author removed it from the extension directory if he/she agrees it no longer works. 

If it is a case of it does not work with version 4.7 but it does work with version 4.6 that will show on the extension page where the supported versions are mentioned. But IMO that is not a 'it does not work', more a 'it does not work with my version'.

I think you should add a little more information about your statement?