Spoiler tags are not supposed to be in wiki pages ever. A wiki page is in effect a spoiler. All the information that is important to a character or story should be written in a wiki page, hence spoilers must be included.
I've removed that spoiler button from the wiki page editor before, but it seems to get accidentally added back in.
I wouldn't use spoiler tags but there are some things I won't do when it comes to things I'll reveal early on a page. There's a variety of things on a wiki character page and everyone who comes to one isn't necessarily looking to have every point of a character's progression spoiled in the first the paragraph or with a character main image that shows a person having leveled up and gained multiple powers, some which can only come from killing another character. It would suck to have all of that spoiled just because you were interested in the Kanji in a character's name.
@AURON570: Back when I was under the impression that plot summaries were supposed to be as detail free as possible, I worked around that issue by formatting certain series like this.
One or two sentences that only give you basic idea of what happened in the episode
A much longer summary that goes through the specifics and details of an episode.
Once the wiki style guide was changed and examples of episode summaries to emulate were going up to like 8 paragraphs long and crazy detailed, I stopped separating the two though. Still, I think a setup similar to the one above would be effective for your concerns. As long as you start with the Plot Summary H2, I think it's ok to add additional sections as you see fit.