For as long as I can remember, one of the big debates in Nascar is whether the Cup drivers should drive in the Busch series or not. If Na$car, (or as a friend on another board calls it, NAPCAR), wanted to make sure the Cup drivers didn't run in Busch races, the answer is very simple. Run Cup and Busch races at separate tracks every week.
When the Cup series runs the 2nd race at California, send Busch to Texas instead of California. In other words, design the schedule so that it's prohibitive for the Cup drivers to run both races on a given weekend. If the Cup drivers want to kill themselves flying back and forth every weekend, let them.
If there was really an interest in making sure that Cup drivers didn't compete in the Busch series, this would accomplish that objective. I don't think it will happen for 2 reasons. #1, there's money to be made by maintaining the status quo. #2, Nascar doesn't now, nor have they ever, thought that the Busch series could stand on its own without the Cup drivers in it. Ultimately, #2 comes down to #1.