It can be difficult to knock out larger infestations of thrips with predatory insects. As far as dealing with anything more significant than the occasional pre-established pest that wanders into your greenhouse, ladybugs are mostly useless in that regard. I recommend removing any heavily infesteed plans, getting yourself a bottle of Amblyseius cucumeris, and perhaps topping it off with some slow-release packets as a preventative measure.
http://www.evergreengrowers.com/amblyseius-cucumeris-1513.html
What temperature are you keeping your greenhouse at? Remember that most insects slow down their activity considerably once things drop below 60F.
I don't recommend this if you have beneficial insects already present in the environment, but spinosad is definitely the best insecticide I've used for controlling thrips.
"There is nothing, Sir, too little for so little a creature as man. It is by studying little things that we attain the great art of having as little misery and as much happiness as possible." - Samuel Johnson