1) Using the "SPOILER" BBCode. This will darken the background to match the text colour, and as such the text must be selected using the mouse to be read. This is suitable only for a sentence or small paragraph within a larger topic.
2) Putting a spoiler notice in your topic title. This is suitable if your topic is actually about something that may spoil the story for other users if they were to read it, i.e. an event in the final mission of a campaign or something similar.
Please make use of one of these two methods if you are posting any kind of information that could be interpreted as a spoiler.