At least your 8 year old grandson didn't write f-u-c-k on the walls. Jesus Christ. I think the fact that he admitted doing it should have been commended by his mom and he should have been shown appreciation by his mom for his honesty even though he wouldn't admit it at first. By disciplining him to the extreme- this mother is setting up this boy for future offenses he commits - perhaps serious ones - where he might NEVER tell his mom about what he does or he'll get disciplined severely. Just my 2 cents from having raised 3 kids to adulthood myself. It's more of a control thing for the mother but it stunts open communication acting arbitrarily like that.
I think a more appropriate discipline would have been to have the 8 year old clean off the writing on the wall and explain to him that even though he was trying to be funny by writing " fart " - it's not funny being destructive to his mom & dad's and his personal property. Not letting him have his friend stay over or writing 100 times " I won't lie " is excessively extreme in my opinion.
As regards your opening title on the thread my elder dad's whipping me with a belt didn't make me a better person growing up- it just made me NOT want to communicate with my dad is all. I figured if I shared my hopes and dreams with him he'd never value my opinion anyway so why open up to him ? His whole goal in life was to make me feel worthless anyway- but I was lucky to have a JW mother who valued me and communicated with me. I give HER credit for me turning out as a positive adult.
So in my opinion - keeping the lines of communication OPEN with kids is the important thing- not dominating them. Teach them, train them- yes- but do it with kindness so you respect them and leave their dignity intact