How To Contain Plants / Plant Roots?

I started out growing some different herbs from seed in a container inside the house that I now want to transfer to a flower box outside. The problem is, while I was waiting for my herbs to get big enough to put outside, a friend of mine had given me a small mint plant. It was little but already big enough to go in the box; well in a matter of weeks, my "little" mint plant has managered to take over nearly half of my flower box! My other herbs are ready to go in, my I'm afraid that if I do that, this mint is just going to "overtake" them and then they'll die :( Does anyone know of anything I can do to sort of contain this Mint and it's roots to it's own side of the flower box and stop it from spreading?