Ok now actually on topic. What I would suggest to solve this problem is a lot in line with what Watson has suggested earlier. Close the road a few hundred metres before the start wherever this is possible and feasible where drivers can warm up a bit. If it's not possible on some stages for whatever reason, then drivers have to do without it, no ifs or buts, it's the same for everyone. Also don't allow spectators to be walking around in that bit of warm up road. Nowadays it's normal for drivers to do warming up immediately before the stage start with public freely hanging around, with little kids running in between cars asking for autographs and it's not a pretty thing to see.
I don't know what is the regulation regarding liaison onboards right now, but those would have to be fully recorded. I am not suggesting someone should go through everyone's liaison onboards every rally, but if any problems or complaints arise, you can access the incident from onboard view as well. Obviously everyone's speed would be monitored as well. Drastic speedings would be taken immediate action against, always, without any ifs or buts. If someone misbehaves, no fines, 10 second time penalties or any nonsense like that, but you are immediately talking about a disqualification and rally bans the first time around. The second time it happens, you are banned for a year at least.
Things change and so does WRC, it simply has to in order to keep existing. Back in the golden 80s in most WRC rallies it was still common practice for drivers to have completely free reign to practice the stages as much as they liked. So you had professional rally drivers in rally cars doing rally speeds, practicing optimal lines for corners, on open public roads! There were no regulations regarding that whatsoever, before the rally or during the rally, everything was ok. For example Henri Toivonen went to practice stages in a long rest halt of Rally Monte Carlo in 1986 to be better prepared for the last leg. And of course as a result of this there were many injuries and even deaths involving innocent members of the public who just happened to be in the wrong place at the wrong time. Just imagine if that was still allowed today! Nowadays that seems often forgotten, now we only mostly remember the Group B cars and crowd behaviour from the 80s. But If that practice wasn't outlawed by the early 90s and the modern recce laws implemented, WRC would have been long dead when writing this message.