Question
Hi
My neighbours tree had tons on babies in my yard =(

How do i remove all the saplings - i have no clue how old they are, i did not even know they were there until i was away for a month and did not mow the lawn.

How do i remove them / preferably permanently  / aka kill them

Answer
You can spray them with a herbicide called Roundup. The foliage will take up the herbicide and transport it to the roots killing the plant. Roundup will kill nay green plant so instead of spraying the sprouts I would take a sponge paint brush and wet the brush with the Roundup and paint the foliage being careful not to get the herbicide on any plant not wanting to be killed.