Root Aphids

Do you remember my small Euonymus Fortui - Emerald Gaity? (Plant Needs Help)

Well, it has now been 2 weeks since I tried to help it gain some strenght. At first, it was encouraging, because I could see new leaves starting to grow, but then, it started to go south again. :(

While some new leaves continued to grow, some of the already existing leaves started to look very unhealthy. They pointed down, even though there was no lack of water, nor was there too much of it. They started to get brownish. Now they are pretty much withering and falling off.
The new leaves are still growing, but they are quite fade in colour. The growth is also uneven from stem to stem.

After researching some more, it appears my poor little plant might have root aphids. :( That is a tiny little bug that lives at the root and gets all the nutrients before the plant can absorb them, resulting in stunted growth, unhealthy coloured foliage and premature loss of leaves. All the symptoms fit.

So I took it out of its pot to take a closer look and, sure enough, there's a colony of little crawlers in there. :( It's unclear to me how to treat this problem. I am also afraid that my other plants will get affected.

So after a good run with us, out it is for Emerald Gaity. Sorry little plant, I really tried...

That said, any advice is welcome! Does anyone know how to treat this issue? Better yet, how to avoid it?