Can we make people wear "spoiler tape"? (spoilers)

[jillily3g]

I am so annoyed with my MIL, who has gloated that she got her book 
for free from shopping at a particular (loathsome) grocery store, 
while I (happily) /purchased/ a US and a UK version, who asked me 
last weekend which page I was on (like it was a race--and then was 
annoyed when I "won"), and who today said, in front of my daughter, 
who hasn't finished, "I knew who would die because he was acting so 
funny through the whole book. I'm just glad it wasn't Hagrid."

Aargh! What was that spell? /Silencio!/ or how about /Stupify!/

Beth