There is no easy solution here.
The ACS Radius server has builtin mechanism's to protect it from DoS attacks (incl accidental) by dropping duplicate requests and any that it hasnt got spare threads for.
There's not much else any AAA server can do. I think maybe you need to look at load balancing to distribute the traffic over several AAA servers.
Or get some RADIUS open source code and build a proxy server that selectively shuts down certain clients if traffic goes over a trigger limit.