We deleted all negative comments in our self-moderated thread because of a recent user made a lot of trouble.
You're free to choose the features you want when posting a new thread. However, since you represent a well-established brand (as you said), you should be aware that posting a self-moderated thread as an official ANN channel is unethical and raises many red flags that might discourage users from joining the site.
If there are posts you consider spam that could taint your ANN, there's a "report to mods" feature you can use to report it to the moderators, and I'm confident they'll make the right decision.
The negative comments are still there is someone wants to do the work. Everything is archived and can be found, just a few clicks.
Deleting posts is allowed but it does look like they're hiding stuff if they are deleting all the negative. I dont know if it discourages people from joining the site because unsuspecting individuals may have no clue what's going on.