Their getting denied access.
Your not able to prevent the 403's from appearing in your logs (actually you may but its not worth the bother).
Your 403 file is 4243 (4.2kb) which presents no real server load.
You could serve them a more custom (almost empty 403) with 1/20th or less the file size.
Their hitting everybody, not just you.
Let 'em burn themselves out.