I use “off topic” on first posts that don’t fit in the forum. For example, a user who asks a question here but actually wanted to ask it in another Discourse forum.
For me, this is also off topic, even if it shouldn’t be moved to a different topic. ‘Spam’ or ‘inappropriate’ don’t fit there any better either, because it’s neither advertising nor offensive.
You could also use it on topics that need to be moved to a different category.