Hi there. I see that you want to use reCaptcha for this problem, so why don't you chose that? Even if you get some login form in there, it should be even easier for bots to enter in a chat ( I am guessing). I guess, if you, in a process of registration, require a mail or a phone to be confirmed, that would be harder to exploit. But still, reCaptcha proved to be the best solution for this kind of problem.
Anyways, feel free to hit me up so we can talk more about this. All the best