Please help i need advise on my twin daughters?

I have twin daughters age 2.5, they have hit the terrible twos :( they are such awesome kids but they are going through a phase of some very fresh behavior, at this age they know that when i say no it means no but they chose to disobey so ive started spanking them. Today they decided to dump out their drinks. i told them to stop, they did it again and i told them one more time and i was going to spank their bum so they did it again, i walked to them and gabbed their arm and spanked them each twice they both cried but 20 mins later repeated the .behavior. i believe it was not a serious enough spanking please help how many spanks? over my knee or standing? and advise on when they both need to be punished.