Maybe just feed them dynamically generated garbage information? More fun than no information.
Depending on the number of simultaneous requesting connections, you may be able to do this without a significant change to your infrastructure. There are ways to do it that don't exhaust your number of (IP, port) available too, if that is an issue.
Then the hard part is deciding which connections to slow, but you can start with a proportional delay based on the number of bytes per source IP block or do it based on certain user agents. Might turn into a small arms race but it's a start.