There is no warning here.
It would be correct to simply penalize the disconcertion.
The proper mechanic is to use the delayed violation signal, and then wait to see if the FT is successful or not.
9-1-3 . . . After the ball is placed at the disposal of a free thrower:
...
c. No opponent shall disconcert the free thrower.
PENALTIES: (Section 1)
...
2. If the violation is by the free-thrower’s opponent only:
a. If the try is successful, the goal counts and the violation is disre -
garded.
b. If the try is not successful, the ball becomes dead when the free throw
ends, and a substitute throw shall be attempted by the same free
thrower under conditions the same as for the free throw for which it is
substituted.