Ten Seconds...

[NCASAUmp]

Actually, I think it's the perfect time to make that kind of call. A scrimmage should reflect how things go in an actual game. If you hadn't made the call, you would have only set the expectation in the future that the rule means nothing. If anything, you helped those teams by reminding them that there are rules, and those rules will be enforced.

And as for a "warning," well, you could have told the offending coach between innings to be mindful of the rule. In your case, their first warning came the first time you called the strike. Nothing wrong with going either way, as long as it only happened that one time per inning. All subsequent strikes were well-warranted.