Just the threat of it worked for mine...but I'd have done it if the warning didnt work (one was using the "sheet" word). Soap is non-toxic - it tastes horrible but wont harm.
LOL
My parents never did this to me but my younger sister will never forgive me for the time I did it to her! I was around 12 years old and my parents went out for lunch and asked me to watch my younger siblings---she was around 7 at the time and told me to 'shut up.'
I've mellowed since then...I don't think I'd do this with my kids but I guess you don't know until you become a parent how you'll deal with situations.