Please require users to start reviews/summaries with SPOILER ALERT when appropriate.

It is already implemented in reviews, but the contributor has to tick the correct box... On the other hand, you can chose to hide the entries with spoilers.

But you are right when it comes to synopsises (i.e. the detailed plot), there is no function nor warning to include it.

Personnaly I do not mind being able to get a detailed account of the movie plot: either I am not intersted to wach the movie but am keen to know the full story; or I missed the ending and want to know.
But I want to have the choice and avoid to be spoiled if I do not want to.

IMDb's plot outlines and plot summaries are not supposed to give away anything about a movie's plot not established within the first quarter or so of the movie's running time, and even in that case, sometimes it is more appropriate to provide only a vague insight. As for IMDb's plot synopses, they are assumed to contain spoilers even if they will not necessarily, and this is indicated on every plot subpage that contains a synopsis item, but even in those cases it is best that the first sentence or so not contain any spoilers, since it will appear on the main page if there are no outline/summary items stored.