You will always "call" obstruction when you see it. But you don't immediately kill the play. (Hoping the following comment doesn't hijack this post - if you disagree with the following, please make it a separate thread!
) Speaking ASA, you should also determine at the moment that you call (and signal) obstruction what base that runner, in your mind, would have achieved without the OBS - THIS is the base she is protected to (don't wait until later to make this determination!).
If the runner achieves the base you were going to award, you essentially disregard the OBS - no award to be made. At most, state "OBS, no award" as mentioned by the previous poster. Many times you'll find that there's no necessity to do anything, or that doing something only confuses the issue.
Also note that there is not always a base "award" (as there is in some rulesets for Smallball), and that it's possible that the base "protected to" and the base "awarded" may be different (example: Runner OBS'd rounding first base is PROTECTED between 1st and 2nd, but not necessarily awarded 2nd if you feel she would not have achieved 2nd base without the OBS).
PS - there's no such thing as a "warning" for OBS, if you mean warning to mean "If you do that again, there'll be a penalty". If a fielder at a younger age has committed an OBS that ended up resulting in no award, there's no harm in mentioning what happened to her coach, in the interests of coaching/training/etc. But at older ages - you probably shouldn't even go there.