Tree roots

I had some large trees cut down a month ago, sycamore i think some 30ft high, 3 in total.. question how do i destroy the roots and if poss the remaining trunk. ( prob 3ft round and about 6" from the ground) ..the guys that cut them said they would be back to sort the trunk but i waited over 3 years for them to cut the darn things down (council) anything i can do myself. Without damaging the soil etc

I wouldn't use weed killer but rather a proper stump killer that will help the remaning stump rot out over time. If you're in a hurry, a tree surgeon will likely have a special cross cut saw device called a stump remover which is basically sanded off with a rotary saw to just below ground level. The roots will take decades to rot away.