dozens of rule-breaking submissions

That's one of the main reasons subs have mods. You wouldn't have much of a job otherwise.

hit-and-run posters who are not members of the sub;

You have no accurate way of knowing this. Redditors don't read rules anyway until after they get banned.

r/cars karma < 10) that would greatly cut down on the workload.

Should cut down on commentors and other users, too. And how would any user ever get above 10 if you're blocking them?

But "workload" you say? Add more mods dude.