First... do not go crazy and put your world vision on a white board that people have to get UP and WALK TO in order to remember what the heck you talked about.
Second... we all have smartphones now. Snap a picture.
Third... I know you're not going to come back and erase your "do not erase"! We're all going to just stare at this mess forever, now with random tiny side margin lists where people are trying to use the edges for themselves to comply with your new rule. You're going to forget you wrote it, and then what? It just doesn't get erased? FOREVER?
So being the sneaky backhanded busy body I am, I like to write "before feb 8" or whatever, always picking a date 1-2 weeks from then, so at least if no one erases my date I know they're not paying attention and we have a time when we can reclaim the space.
Other times I just really feel like erasing it. But if I'd done that, I wouldn't say it here.
I had a coworker who was always making shared files for documents we were using, and sometimes he'd create a folder called "newest versions". Except that folder was only good for like a week, and then you had to tribal knowledge what the REAL newest version was. I tried to explain that it'd be better to make a folder called "old versions" and move those files in there, because once you're old you're old. But you're only new temporarily. Same with that white board... nothing is important forever. No one wants to take the time to DECLARE that something that was once important, isn't anymore. So save us all some time, cut to the chase, and admit that it's all crap to start with.