SharePoint - Allow members to share the site and individual files and folders

Hi

I understand that by default when a site collection and sites are created, the option Allow members to share the site and individual files and folders. is ticked. This allows members of the site to 'Share' files. This feature overrides the existing permissions and creates unique permissions on that file. It gets messy fast!!

Now we are in a situation where one site collection has over 100 sites. Within these sites members have shared documents they shouldn't have. 

My first question is, can I turn the share function off at site collection level or does it need to be done one each site? 

Once it is turned off does it make the links that have been shared invalid or are they still active? 

Thanks

Karen