"Hiding" spoiler text is very easy to do, as long as you use the "Reply" function and not the Quick Reply. The Reply function gives you access to all of the post formatting tools.

To hide text: Simply select the text that you want to hide, click on the text color icon (3rd from the right - if you hover the cursor over it, you'll see "Color some text"), then select the color that is 5th from the left (the light cream color).