The fcq's round robin multiplexer actually rewards abusive users. Queue consumers scan for a call from the roving multiplexer index to the lowest prio ring before wrapping around to the higher prio rings.
Let's take a fcq with 4 priority rings. Multiplexer shares per index are 8, 4, 2, 1.
All well behaved clients are operating in ring 0. Bad client floods the server and drops to the lowest prio. Unfortunately the service order gives 8 shares to the good clients, followed by 4+2+1=7 shares to the bad client.