If it's one post and they're actually engaging with the community and not just dropping the ad and leaving, then I might be okay with it. Even if they're not offering free slices.

The ones that bug me things like the constant ads for comedy events, where they blindly blast it across multiple subreddits, and continue to do so even when they don't get any engagement.

Reddit has pretty good guidelines here: https://www.reddithelp.com/hc/en-us/articles/360043504051-What-constitutes-spam-Am-I-a-spammer-

And while it's no longer updated, the self-promotion guidelines are really good: https://www.reddit.com/wiki/selfpromotion