[tor-relays] What to do about port scans?

I wouldn't have thought that the Tor network was fast enough for port scanning, but apparently it is. I have recently seen a rash of SSH port scanning (or so my ISP reports). What can/should I do about this?

I know I can limit the rate of connections using iptables. What's the consensus on this? Is this considered advisable, or a breach of expected exit node behavior?

Do I have any options other than iptables to restrict the rate of port 22 connection attempts?

Thanks.