Not sure whether your version of Lr has the "auto mask" option for the adjustment brush but if it does make sure it's checked and it gives a much better selection. Alternatively if there is an erase brush option use it with a harder edge to brush out areas you don't want selected. Easier to do this with the "show mask" option - again not sure whether you have this option. I know how to remove halos in Ps but not in Lr.