Stop Mint From Spreading Through The Rest Of Your Garden

A few years ago, I planted a packet of mint seeds behind my house.  I was surprised to find that they came back the next year in abundance!  They are almost like weeds in the way they spread.

To stop new growths from spreading further and getting into the grass, I put small, decorative bricks around the mint plants.  It helps, but the root system still spreads outside the brick area and we still have to cut a few mint plants down at the root system.  We have clay soil and I find Mint to be a very hardy plant, that can handle almost anything if it is planted outside.