I've had an archer in my group who used a laser-tool to measure the distance between the stake and the target in my last 3D-archery tournament.
He used it at the first target but understood the group's "body reaction" that it was an absolutely outrageous and unruly behavior. Therefore he didn't do it again.
The tournament of course had covered the rule:
The tournament consist of 29 regular targets with unknown distances and 3 "special targets" also with unknown distances.
Not allowed equipment: (...), distance measurement tools, (...)
However, I wonder how to deal which such a person if he wouldn't have stopped? What should I do?
Note: One of the key facts that make 3D archery unique is that you don't know the distances which you shoot. This is an absolute core rule to establish an "hunt like" environment.
Read more at Wikipedia:
It is most common to see unmarked distances in 3D archery, as the goal is to accurately recreate a hunting environment for competition.