The relay software is quite basic and it doesn't have a moderation dashboard. You literally have to use SQL to search for bots and since relays allow anyone to connect you get thousands of bots joining.

Imagine trying to find each individual bot. There's a big chance you will remove a legit account. That's the problem with allowing anyone to connect for free.

The paid relays don't have that problem because it cost sats to join the relay so bots don't want pay.

I think it is impossible to put good metrics to block a user to post its first post. And when one does it, this relay is not able to accept new users.