I’m assuming the above posts, should have smiley’s after them, including the OP.. leylandii, which those look to be are pretty hard to kill by topping. Though drilling holes in them and stuffing dead branches is probably going to be a good effort. Even if you top them, they will simply grow from just below where you chopped them off, as they just grow vertically.
In all honesty I chop them down, as there simply nasty and doing you no good (ppl plant them to get privacy), and plant a nice fruit tree (or two) instead. Plums tree grow remarkably fast and give huge crops.