You asked me to include BBcode for strike-through text and spoilers, so here we go :).
When you write a message you can click on the buttons on top of the text field to include the code:
SPOILER: Will hide text so you have to click on "show" first to read it. For example this should be used if you write about the story of a movie others might not have seen yet. Please use it from now on!
This is an example.
STRIKE: Will show text as strike-through text. There can be different reasons to use it, for example when you edit a post and want to make clear your old text doesn't matter anymore. Or just for fun.
[strike]This is an example.[/strike]