Writing a letter (citing statutes or not) is fine, as a notification device. If it doesn't stop the unwanted behavior you'll have to find the money to hire a lawyer and litigate.
Despite the LLC being removed, there's apparently still another entity filing paperwork with the SoS as a for-profit limited liability company, having our previous entity name; and the registered agent is the spouse of a former Director.
I'm still not getting it. Are you saying that your government entity dropped the LLC from its name and now some other entity is using the same name followed by LLC?
Is that "entity" also a government agency?
If not, I don't see why you would think it illegal to use the name.
Anyway, here's the entire Colorado statute regarding LLCs. Can't imagine why you haven't already found it.
Colorado Limited Liability Companies Laws - 2020 Colorado Revised Statutes Title 7, Article 80 - Limited Liability Companies :: 2020 Colorado Revised Statutes :: US Codes and Statutes :: US Law :: Justia
If there is nothing in there expressly or impliedly prohibiting the use of a name that happens to be the same as a government agency, then you'll have to find other grounds to object to it.
Example: There is a town in Colorado called Castle Rock, a government "entity" right?
While I couldn't find a Castle Rock, LLC, I did find a Castle Rock, Inc listed on the SoS business search. ID # 20218021538.
If you were employed by the town of Castle Rock you would be looking for a law that prohibits Castle Rock, Inc from using Castle Rock in its business name. Is that what this is all about?
Because if it's not, then you'll need to find a better way of explaining what's happening (including the name).
If you can't do that then there is nothing more we can do for you here and I suggest you take your issue (whatever it may be) to an attorney.