I agree with Sjeord. Plant mint in a pot and sit it on a concrete surface. That's what I've done and have no problem with it 'travelling' all over the garden.
I am, of a sort, a warden. I have for many years kept an invasive species at bay, imprisoned in five pots with their rims above the surface. For 13 years the Gooseneck Loosestrife has yet to make a jailbreak, I suppose I would treat the mint with the same conditions, a green pot for a prison and feed only bread and water. Of course the bluejays always grab the bread before the plant. Jerry Ref: http://www.gardenstew.com/blog/e10953-2 ... rison.html
From everything I have read and everytime I myself have planted mint, the correct way to plant it so it does not invade in indeed in a pot above ground or in a pot submerged!
