While a nice ground cover and a brief flowering period, Lilli's spread like wild fire and are stating to overrun the garden. Is there an easy way to Kill them off without bothering the other plants? Would tilling the soil in the fall or before they pop up in the spring help? I have heard that the cut roots will just continue to grow.

You have to remove completely all pieces of the plant. I thought I had removed all of the plant that I had growing in my yard, but lo and behold this year a few had managed to reappear. This plant is like mint - if you leave one piece in the ground, it will grow again.

Ujay is right. The stuff is the mint of the bulb world. (tuber actually) I love the stuff but know that to introduce it is to surrender to it. The best time to dig it up is now, when the leaves are obvious. You will have a better idea how successful you are.
I planted it in only one spot of the 56 acres I garden, under the yews at the daycare. I wanted something pretty and effortless. It took three years to fill in a bed 4' by 75' with an origination of 75 plants.
Offer what you dig up to friends. They will think you generous for a while (until they too are over run)