You can hide text like this:
Spoiler text.[glow=black,2,300]Spoiler text.[/glow]
Or even cooler like this:
Spoiler text.[color=#ECEDF3]Spoiler text.[/color]
Use #F6F6F6 instead of #ECEDF3 if your post is on the lighter blue background of the default forum color scheme.
The two examples above obviously won't show up correctly for anyone using alternate color schemes or themes, light text on dark backgrounds, high contrast modes, simplified reading modes, etc.