There's really nothing to do. Based on the limited
logs, it looks like someone was just looking for open TCP port
22 (ssh). You can't really block the scans by source since you
don't know the source address (because Tor). You could prevent
connections to port 22, but that would prevent everyone else
from using ssh through your exit, and also, it wouldn't stop
port scanning of any other ports allowed through the exit.
I'd just explain you're running a Tor exit, and thus you
cannot identify the source of the scan.
As common as port scanning is (and has been for as long as
the Internet has been around), I'm surprised providers still
worry about it this much.