Yes.

Yes, too much salt will burn the roots. If you can, dig it up, give it a water bath to get rid of any salt build up & replant in a different location. If you must have it in the same location, thoroughly flush out salt deposits with water & replant.

just be aware that too much salt may harm the balance of the soil and affect other plants you wish to grow. how big is the sycamore tree?
You could ring the tree to kill it. gouge an 1-1.5 inch wide strip around the whole of the tree about 6-10" up from the soil. you should find the top layers peel away easily down to the heart wood.
I have killed many young nuisance trees in the garden and nature reserves when needed. it doesn't harm the environment either.