These are mealy bugs. They're incredibly difficult to eradicate, due to their habit of finding crevices to hide in. Even when you do get rid of them, if there is one egg or juvenile left, you're back to square one.
I've had cases in the past where I had to throw out all the plants because of an infestation, after battling them with everything I could think of, for ages.
So, first of all, isolate this plant. Don't touch other plants without washing your hands thoroughly. Check other plants in your collection to see if they are infected too, and quarantine any new plants that come in.
There are some sources that have had good luck (reportedly) using a q-tip with rubbing alcohol on it, to get the little rotters out. This may or may not work in this case, because of the tight quarters. I might use a spray with insecticidal soap and rubbing alcohol, and spray to runoff, to make sure to get in the crevices.
Alternately, a systemic product might work, but I can't speak to that as I've never used it.
Apart from that, keep at it. Don't relax, even if you think you've got it beat, because at the slightest opportunity, they'll be back.