Well you're doing a big mistake by tolerating "one time offense" because by that you basically allow anyone to go unpunished while being toxic. Two premades just ruined my game, they didn't respect Draft pick autofill, one took jungle away from our jungler and the other took mid away from me. I'm sure this is not the only case and that has happened to many people.
It's time that you think about how much you acutally tolerate, by tolerating "one time offense" you allow toxic people to make this game worse and then toxic people tilt other people which are completely fine otherwise and that is how community becomes even more toxic.
Pointless to say these 2 guys were the only reason our team performed so bad in that game, even more when I told them I'm gonna write about that on boards, they were like "do it, we don't care, haha, still waiting for ban" etc. which is only a proof that they think they can do whatever they want and they expect to go unpunished (which is exactly why they should get immediate punishment).