Like a few people who've commented here, I've always foudn that being ignored is a harsh form of punishment - and I know I've done wrong. And I don't like it either.

Something else that I've experienced is being given a l-o-n-g task to do, as my Master knows I have a criminally short attention span. For example, I once had to mow our lawn with a pair of kitchen scissors. I love the first 5 minutes, but the next 1.5 hours wasn't so good.

So I think picking on a personality trait like that in a sub can help to find an effective form of real punishment.