Just like you moderate every forum or discord server or Reddit page or whatever.

In Telegram some public groups have a captcha bot. Every user joins without permission to post by default, the bot posts a captcha along with a link the user can click to open a DM with the bot where they can then send the captcha answer. But that’s a mechanism working on top of another, as telegram account creation requires a phone number. Maybe the best approach is just layering in general.