Update: Apparently the stewards have said they had 1,200 reports of track limit infringements to review. They sound as frustrated about them as we do. At the end of their statement they said something like, "As we have in the past, we recommend this track add gravel or grass on turns 9 and 10."
Apparently other series that run at the track would become more unsafe if grass or gravel was introduced at those places. It’s plainly F1 drivers taking advantage of a situation when they have been warned not to do so.
Adding grass or gravel for the F1 weekend and then repaying the surface for other series seems to be the only viable solution. The track should do that next year and send the bill to the GPDA.
Aren't we infantilising F1 drivers then? "If we don't put something physically in their way they won't obey the rules".
Other series have figured it out - MotoGP paints off-limits areas green and they put sensors on the edge. You get too many infringements you get a long lap penalty which costs you a few seconds.
I think this race worked well and they need to do the same at future races, and build consistency. Then people will learn not to take liberties.
If the enforcement overhead is too big, add a fine to the penalty as well: you cross off the track, you get €500 fine as well. Then use that to pay for extra stewards (or set up an automated system).