If someone cares that much about a spoiler, they should know to either get to the movie quickly or avoid the obvious places it will be spoiled.

I think the best courtesy is to keep spoilers out of things like post titles on reddit for a long time/maybe indefinitely.

I personally don't care about being spoiled, so that's where my motivation comes from lol. I have either forgotten spoilers or enjoyed movies that have been spoiled every time.