One way of disabling downloading a file is to remove it from the server - this can be done easily by using the time stamp on the file (if it's been set correctly, which it usually is).
For example, you might remove files a month old in the current working directory with:
find . -mtime +30 | xargs rm
NB: The above code is not certified for production use; ensure that it deletes only files you want deleting before running it anywhere.
Mark