I'd say just make it so you have to play 5 normal games minimum with a specific champ to be able to play that champ in ranked. Just because they waited a week doesn't mean they actually learned how the champ in question works.
It's really not that hard to pick up a kit once you get a basic feel for it, so I feel like 5 games is a little bit overkill, but that might just be me so I don't know.
The first time I played Rek'sai I didn't realize that the bonus fury on her passive only triggered if you burrowed and then surfaced again within 5 seconds to trigger it, so I was finding it hard to clear with her because I was missing out on a ton of healing due to that.
Things like this make a HUGE difference, and could be caught very easily in 5 games I'd say, or at least I like to hope that if people were dying every single time they tried to first clear that they wouldn't bring that champ into ranked, but who knows.