There are many columnar trees in the market today that are great performers. We have a columnar tulip poplar at the office called 'Arnold' - they line our office entrance if you would like to see them. You might also look at Columnar English Oak, 'Crimson Spire' Oak, London Plane Tree 'Exclamation'. There are some apartment buildings on Country Club between Cox Mill and Canton that have a group of Columnar Sweet Gum 'Slender Silhouette' - it still has the spiky balls but in less quantity. If you would prefer an evergreen, take a look at 'Taylor' Juniper. These are all pretty close to the same height as your Lombardy poplars.