My suggestion would be dig very deep and remove each piece of root if the ground is not to hard. Round up will not help you, because these are bulb type plants. I did battle with wild onions and the removal process took years. You have to remove all roots you can again and again. It's going to be a huge job.

Use a perennial fork to pull them out roots and all. Good luck, sounds like an ugly job.

I would think any chemical strong enough to eradicate such hardy tubers would have a toxic effects on anything else you planted there.

Maybe a post of "free daylillies - first come, first served" in the other forum will work. When they show up, you hand them the shovel (just kidding).

Have you tried freecycle? You could probably get rid of a bunch of them if you do.

Here is a link that might be useful: Find your city/county

I had a war with my daylillies out in front of my house a few years ago. I loathe the homebuilders choices for landscaping when they planted a mass of dwarf irises, thorn ridden shrubs, and daylilles in front of my house. I should have just bribed the landscapers to spare me the headache and not plant anything.
Anyway, I just went in there and pulled out all of the daylilies and irises. It was a job in itself to dig up the roots in heavy clay soil. I think I ended up breaking a shovel in the process.
If you can soak the soil or do it after a good rain in winter, your luck might be better in extracting the roots. I did mine in January and the deeper I dug to get a grip on the roots, the easier it was get them out of the ground.
You can post a sign "Pick N Pull Daylillies" or spread the word over at the Daylillies forum.