Riot's working on it, but it's difficult, since the bots keep evolving, and there are many times more bot programmers than Riot's behavior team has. Furthermore, just about any number of false positives here would be unacceptable: how do you ban away all possible bots with zero real players getting caught up in it?
I'm curious how you know they're bots when you're still in champ select. I wouldn't assume anyone who is silent in Coop select is a bot: I usually am myself. Unless I'm jungling I don't really care what lane I get in Coop, so I'll spend my time fiddling with masteries for the new champ I'm playing Coop to try to learn. I further assume that in champ select, bots act just like I do, saying nothing, picking some champ after a few seconds and waiting to start the game.
Honestly though, while I still report them in-game (please do, it's part of how Riot actually finds new ones so they can restrict them more), I like playing coop with 1-2 bots on my team. I find it makes for a much more fun and challenging game.
Sooner or later, the bots will be well-enough scripted that even in-game you won't be able to tell the difference. At that point, unless I'm working for Riot, eh MFA: https://xkcd.com/810/