Yes, for not-fun punishment, being ignored is the worst. It really truly should only be used in situations where the sub has been totally out of order (in my opinion, of course).
Any other punishment for not-fun should certainly be something the sub is not going to get any pleasure out of - sometimes, just the dom/me telling them how disappointed they are, how hurt they are, will work very well. That can be dire. Standing in the corner type stuff for misdemeanours is pretty good, too, I think. Boring. Gives you lots of time to think...
Fun punishment - well, mostly, that's going to be pain if you're into pain, isn't it! I had one fun punishment for cumming when I shouldn't have - pair of geisha balls, still don't know how you say that in English, on in the front, one in the back, 2 hours, go about my normal day. Damn! serious turn on (but not enought to cum and knowing there would be no orgasm at the end of it) and nasty pain from the stretched string at the same time.