Rhubarb should work well, if you get a large kind. It has huge leaves that cover the ground around it. I personally consider it ornamental, including when it goes to seed. It should smother out anything else underneath it. Some rhubarb gets quite tall (it may not get huge on the first year, however). I've never grown rhubarb from seed (let alone scattering them), but it does produce lots of seeds. It's not invasive. Rhubarb is a perennial.
Burdock looks similar to rhubarb and might also work (in my opinion, rhubarb would be much better, though). It may also grow well from seed. However, it is invasive (I'm sure it would make appearances in your garden, and your neighbors'), probably doesn't always cover as thickly as rhubarb, and it will produce burrs that may get stuck in your cats' fur. Burdock has many uses, but it was a common and highly invasive weed in my neighborhood years ago back when I had a long-furred cat (too bad I didn't know what it was and what you could do with it back then). It seems to have vanished from our yard, however. You can buy it. Burdock is an annual. So, it'll have to grow again every year. I'm not sure if it'll grow to size fast enough to make a difference, but it might. However, when they reseed, they may not appear in the same spot. So, you'd have to collect the seeds yourself every year and plant them again. All in all, I'm not suggesting burdock so much as I'm saying it might possibly work (with drawbacks).
You could always plant some currant bushes over them. They should grow and smother them in time. They can get pretty tall, although you should probably cut them back once in a while. Currants will spread horizontally through the years, although they may spread by seed, too. I'm not sure how they'll work in your climate, but this is how black currants work in my climate (which is more on the arid side in the summer).
Spearmint, with its rhizomes, may also work. Comfrey may also work. Chives may work, if you plant enough of them. None of these are 6' tall, however.
Irises, though not tall, are flowers that may smother them out eventually, with their thick, tuberous roots (don't expect results the same year, though, unless you plant a lot of them).
Snowball bushes (not sure what they're technically called) are pretty tall and nice-looking, and depending on how you let them grow, and perhaps the variety, they can potentially cover the ground quite well with their branches. They are wide in all directions, though (so you may or may not want to put them a little ways away from the road).