Silly question perhaps, but how long have you been using Usenet? Every newsgroup I've ever toddled into has it's own preferred etiquette, and much like any other discussion fora, it's going to blow an ill wind if you start using it for announcements instead of discussion. DH has a clear warning in their TOS about such things:
"Electronic forums such as mail distribution lists and Usenet news groups all have expectations regarding subject area and appropriate etiquette for posting."
If you didn't investigate those expectations it's roughly the equivalent of shooting yourself in the foot. Checking through Google, you posted several different announcement/spam style posts over the past months and every time received a slew of very hostile replies, including several requests from as far back as March to "Please take your mini-chinese-buggy spam foo somewhere else. Thank you very much." If you don't bother about such requests made to you on public forum by its participants, you're definitely bucking for trouble.
In DH's anti-spam policy it's specifically stated:
"Predominately promotional messages must not be posted to bulletin boards, discussion forums, guestbooks, Usenet newsgroups, or any other similar service unless the operator of that service explicitly allows such messages. In the case of Usenet newsgroups, such messages must be specifically allowed in that group's charter."
Now I realize you're saying you're not a profit-based organization, but the posts you made definitely are of a promotional nature, and unfortunately, you signed the contract not to violate those policies.
Unfortunately, hosting providers can suffer greatly if immediate (..and often draconian) action is not taken against offending users, including, but not limited to, blacklisting of their servers - and that affects ALL OF US other customers here.
If you'd have managed this as a single offense, I'm sure it would have warranted no more than a slap on the wrist, but seeing as the abuse was committed over a series of months and you had several opportunities to discontinue your abuse, I suspect you have either a) a long fight on your hands, or b) a short walk to your next hosting provider.
In any case, best not to bitch-n-holler too much and just chalk it up as a learning experience.