Green fly in my greenhouse already!

I bough some mint plants at a plant fair about a month ago.... it had lots if white fly on it, washed them off and split them up to make extra plants. Its the variegated variety which I haven't got.

I've been very vigilant on what plants I've brought in but this morning I've spied a couple of green fly.  
Now what to do - i've squished the half dozen i could see. They are all around the mint. They have gone out....but the greenfly were on some tomato plants that were next to it.

Im tempted to sacrifice these black cherry toms...  I really don't want to battle with a ton of greenfly all summer.