Scale bug infecting a dozen plants, tried everything...?

Tried black ladybugs, tried washing in washing up liquid, I am on the verge of throwing them out. The plants are too large to go over every leaf - although I tried this anyway - some are over five feet with more than several hundred leaves. I could dunk them as a whole in something cheap - that is, something I could fill a tank with - but I have cats so nothing poisonous to them. Or I was thinking that I have them in a lean-to to conservatory, at present warm cosy, so I wondered if leaving them outside would kill the bugs. Yes, It might do away with the plants themselves, but I am on the verge of throwing them out anyway! Help...?