[bossref]

The way I learned it was that if a defender
is waving hands to DISTRACT then it if OK.
If the defender is trying to totally OBSTRUCT
the visual field of their opponent, this is
unsporting, and a T should be called.

And yes, I agree with giving a warning
if they are younger/inexperienced players
who could use some instruction.

If you sense that the player knows exactly
what he/she is doing, T them.
They will learn very quickly that this is not acceptable.