Introduce a "spoiler" tag.

<spoiler>
Here something that really spoils the movie.
</spoiler>

This should be automatically replaced in HTML with something like:

<a href="javascript:show(spoilers)">show spoilers</a>
<p class="spolier">
Here something that really spoils the movie.
</p>

and not displayed by default, only after clicking the "show spoilers" link.

Implementation as a django template filter + add <spoiler> as an acceptable tag.