Unfortunately, as you know, day lilies can be very difficult to control.  If the area was smaller, you could keep digging out the bulbs everytime they resprout and over time you could remove them all. But, in this case, you might need to resort to using chemical options. 

Using glyphosate (Round Up) can be effective if the daylilies are actively growing. Since glyphosate is most effective when taken up by leaves. Directed application (spray bottle) on just daylily foliage is advised. If you use glyphosate, make sure to read and follow the label direction.

Another option, is to cover the bed for a length of time to smother the lilies. Cut the daylilies down and cover the entire bed with black plastic or cardboard and then mulch . The bed needs to be completely covered, even out beyond the edges of the bed to keep the unwanted daylilies from receiving sunlight. Weight the plastic down or stake it down so that it will not blow away. Be sure the mulch is at least 2 to 4 inches deep. Keep the plastic over the daylily bed for at least nine months.

Check out the following for more information on controlling day lilies:

My Daylily Is Out Of Control - How To Get Rid Of Daylilies (gardeningknowhow.com)

Plant Invaders of Mid-Atlantic Natural Areas publication:

https://www.invasive.org/alien/pubs/midatlantic/control-herbaceousforbs.htm

Good luck with your efforts! Robin