Maybe I just happen to be in games where this doesn't happen. And I feel like the advantage you can get from this is really minimal. Arena Elite seem to reflect true strength (at the moment) pretty well._ Arena + turn-based seems incompatible due to the possibility to slow down losing game while accelerating winning games. This is especially true at the end of the season where all turn based games in progress are removed from Arena.
In any case, disabling an entire mode seems like a real strong hammer as an attempted solution.
You don't need to "solve the problem 100%". You just need it to happen rarely enough, with not enough advantage. Boardgamearena doesn't exclude questionable behaviour anyway so its strange to try get such a small point to a pristine state. For example, there are real-time players who will quit the game when they are losing and their opponent runs out of time. As a result the game is cancelled. This is a great advantage compared to players who don't do this (a player can get back into positive time by playing faster on future turns).
I want to reiterate others' suggestion: separate real-time and turn-based arena ranking. Treat them as two different games.
This way, the choice is left in the players' hands. The ones who think turn-based is a problem can just choose to not play turn-based arena and have the same result as the current implementation of disable turn-based arena.