Hey TJGuy, as far as my Spanish is concerned, you used it correctly but "de acuerdo" was not the best phrase to choose in that situation, hence the reaction you got.
I think the Guardia Civil simply wanted more of a serious and respectful answer out of you like...
� S� se�or, lo siento. No volver� a pasar/ocurrir.
� S� agente, lo siento. No volver� a pasar/ocurrir.
� Lo siento agente, no volver� a pasar/ocurrir.
� etc...
You see, "de acuerdo" is by nature very "whatever you say"ish (like in your first 2 examples above which are more relaxed situations) and the Guardia Civil here can be very "chulos" and authoratative, especially those on highway patrol.
It would be like getting pulled over in the USA by highway patrol and saying "whatever you say" when he told you not to do it again. "Whatever you say" would NOT go over well with most of the bulldog looking highway patrol officers I know
Your "de acuerdo" was a lax way of agreeing with him (like saying "whatever") and he wanted you to give him an appology and a "sorry sir, it will not happen again".
Basically, he responded "No, not whatever you say, don't let it happen again". He wanted to hear you say specifically "it will not happen again" ("no volver� a ocurrir").
Just my take on this.
Saludos,
jer...
p.d. Hmmmm... "no lo repitas de hacer..." seems like a weird construction to me, sure he did not say "no lo vuelvas a hacer"?