My son has decking across the bottom of his garden and foxes are quite happily living beneath it. However, they have excavated entrance and exit holes in the front ( house-facing) section of the decking which my son finds a messy and unsightly problem. He accepts that the foxes are entitled to live somewhere but wants to divert their comings and goings to the sides and rear of the structure.
I was thinking of some sort of physical barrier, ie, metal rods 4 inches apart thrust a metre deep across the whole 45' wide decking. This would be very expensive as each rod would cost about £4.
We live in the depths of the country,but don't have any problems with foxes.However,the main part of our garden is lawn,bordered by flower beds,and as Murdo's link says they prefer cover from bushes and plants.
Murdo's link offers easy,effective and ecological advice.
So when your teenager next wants a wee,push him out into the garden!