Warning for delay

[JS 20]

Worked a game earlier this week...there's a foul and defender B1 winds up w/ the ball. He takes a jump shot well after the play has been stopped by a whistle. He he shot at the other team's basket after he wound up w/ the ball. A few minutes later, he winds up w/ the ball while on offense after the play has been stopped and he shoots it again. I go up to him and say "hey, we're not playing a college or NBA game today, do me a favor and when you wind up w/ the ball after a whistle, just give it to me or my partner and don't shoot it, ok?". He says ok and we go on. That was the second quarter. At the end of the third quarter, he winds up w/ the ball around the free throw line in the backcourt. Time expires, 3 or 4 seconds elapse and he throws the ball at the other basket and it bounces off the wall on the other end of the court. I go have another chat with him, and figured a warning for delay would be the most I could penalize so that's what I did. My questions are 1) can I warn for delay like that between periods? 2) What would you have done differently?