Yep - it sounds as if they are in control!!
Obviously, telling off and smacking don't work. In fact, their antics sound to me like classic attention seeking behaviour - and boy, is it effective!! 6-10 trips upstairs every night - wow!
here's what I'd suggest: instead of going bananas at them for being horrible, reward them and praise them for being good. Give attention for being good. There will be some times you need to step in and prevent floodings, of course, but in the main they should find they get your attention for being nice and co-operative. So, you take them up to bed, tell a story, and then say you will come up again in half an hour for a cuddle/sing song/story/quiet game of cards but only if they behave nicely. Keep your promise if they keep theirs. If they continue being good, they get a star (a shared chart, probably)...and five stars in a week means a special treat. You can never withdraw a star once won.
Give lots of love and praise and adoration when they are good - really OTT, it's fine.
Be consistent - you need to try this plan for at least a few weeks, and to adopt the same strategy in the day, too. It will not work if you crack.
(I speak from experience. Also a good book is The Good the Bad and the Irritating by Dr Nigel Mellor, for parents of attention-seeking kids. It's puboished by Lucky Duck Publishing). Good luck!