To cope with a couple of dozen plants infected I remove by hand .Look underneath the leaves carefully because they seem to be the same colour as the leaves and can be difficult to spot until used to by a little practice.
A garlic spray is sometime used by big growers.
The advantage of removing by hand on a daily basis , if possible time wise, is that the whole plant can be examined for any other problems.
I find the greenfly problem seems to last for a short period ,and then thankfully it seems to stop.My experience in the last three years.
Perhaps the leaves become tougher as they get older
If in flower they can be pollinated at the same time just by using your finger tip.
This year I have bought a bottle which is diluted to the instructions given.
To be fair have not tried it yet as no greenfly .
The cost was high for the volume of contents and it might be worth searching on the web for help how to make your own.
Perhaps other members have got their own muck and magic spray which they might be willing to share.
A devil aren't they? I put a squeeze of squeazy into water in a half litre sprayer and it cleared aphids from my currants. But it only kills ones you spray, it is not persistent. Pyrethrin is another good insecticide, unlike soapy water it lasts a few weeks, finally degrading in sunlight. It kills all insects. It is toxic to cats, but relatively safe for humans, but asthmatics might do well to avoid it.
The problem with chillis indoors is the absence of predators.
Don't put pyrethrin on the tree, it'll kill pollinators, ladybirds etc.