So how about limiting number of possible simultaneous showdowns to lower value? Now it's 50.
You ban this same cheater again and again, but current system allows to jump him with new account into legends in 3 hours using hacked tickets. Check diagram. It shows how many stars every rank have. So if he run 50 showdowns he not only jump into legends from 25th rank with streak after he finish all of them, but he have also 50-41=9 extra wins in the legends. So it automatically puts him into ~20th position in legends.

Just limit that number to 10-15. No one needs so many. This parameter called "ranked_match_limit" on pvp server, just change it