On the serious side,
Banning someone for giving out spoilers is the lamest, dumbest, shittiest, most idiotic thing to do!
So, lets start from the statement that giving spoilers is griefing.
In order for something to be griefing, the other party needs get annoyed, irritated etc. Now, if someone gives spoilers about tv show, or something else, some people will get irritated, in which case to them it is griefing, and some won't, in which case its no offense at all. Now, we already have a split conditional offense.
Should my neighbour be arrested for having bbq that produces smell which bothers me? or should I just close the window? Now, what about those who do find it offensive? should a man giving out spoilers be banned , or should those who find it offensive just mute him?
Offense he did, was personal, and it CAN be solved personally. Global punishment is only for something that that individual cannot resolve himself.