Best thing is to cut it down. If it isn't yours and you are found out there may be big trouble. It is said that if you hammer copper nails into the trunk it will kill the tree, but it would still need cutting down afterwards.

Ive heard about nails and vampire's but never in a leylandi,

The way to do it "Is to cut the tree down to ground level and drill as many holes in the stump as you can,
fill these holes with stump rot or simular chemicle rotting compound, (the garden centre's in the uk will have such stump rotting chemicle)
This will kill the roots but will not harm the soil.

Thank you to Dungy and Steragram for your comments

You don't need to use a stump killer for conifers. The stump just dies off anyway, unlike deciduous tree stumps that may sprout again.

If you've both a life time to wait and plenty of garden space then leave the stump to rot,
if you want to speed the rotting up use the stump killer.

I cut it to ground level and place a rock or an ornament on it.

We had a 30 ft leylandi cut down and just left it , it looked quite ornamental amongst the surrounding plants but it did not take long to rot away the tree remover did do a good job of cutting it as close to the ground as he could .

I tried putting a bowl shaped pot on ours, but sadly it wasn't quite level so it looked odd rather than decorative.
What a lot of conifer stumps of one sort and another there must be all over the country!