Predatory insects are my favorite means of control and I typically kill aphids with a small container of ladybugs and a bag over the plant. The ladybugs can't fly away and greedily annihilate the aphid infestation. Unfortunately, waxy scale (Diaspididae) has few predators. There's a parasitic wasp that can be used, but obtaining them as a homeowner rather than a grove manager is difficult. There's not much else to choose from.
As Tim notes in his comment, Diaspididae is very resistant to pesticides. The females' waxy covering protects them from contact poisons and I've yet to have much success with systemics. Diaspididae crawlers can be killed by contact poisons, but the females produce so many offspring for so long that wiping out an infestation completely requires total concentration. I've never been successful at that, probably because I've never been able to completely coat the plant in poison. One little spot on a leaf left untouched and some set of crawlers will survive to the next generation.
I've had the most success with a combination of crushing them with my fingers and a contact poison such as rubbing alcohol. That's known to be incomplete because, again, you just never seem to get every scale and every surface on a large plant.
This leaves environmental controls. Here's what we know about our two combatants - Diaspididae and Gerbera.
Chrysomphalus aonidum has a preference for humid environments and cannot tolerate freezing temperatures.Gerbera:
Can tolerate some frost but freezing temperatures will kill plant to the roots.Hmmm. The freezer may well be my friend. It looks like we need to do some experimentation with freezing infested plant cuttings to see how long it takes to kill the little brutes.