This is a tricky issue and varies depending on where the main areas of use are and you will need easy access to your logs to find out where the problem is coming from.
Trace the worst abusers
And contact their ISP.
Block external linking
In Apache you can block thing through your .htaccess file.
As far as anyone can tell the revisit metatag is largely ignored by search engines and they index at will (which, on popular sites) might be once a day which soon mounts up. One of the best solutions seems to be to use the If Modified Since header which tells anything indexing your site that the page hasn't changed since last time and so it can use the old information:
