The best flag to use for nonsense post is actually either "not an answer" or "rude or abusive". The rationale for which is nicely explained by Shog in this main meta post:
This is an utter waste of time. There is no meaning to the post! It's
VLQ, it's abuse, it's Not An Answer, heck it might even be a spammer,
testing the waters... There's no metric you can apply that'll narrow
that down, because there is no meaningful content to apply metrics to.
So pick the flag that speaks to you. I'm partial to "rude or abusive",
because enough of them immediately delete and lock the post, which is
handy in those rare scenarios where someone's flooding the site with a
lot of these... But VLQ or NAA work just as well in the vast majority
of cases. The important thing to remember here is that when the post
clearly means nothing, you shouldn't be wasting too much thought
trying to decipher it; flag it and move on with your life.
That said, the only difference between spam and R/A (rude or abusive) flags is that R/A flagged posts are never used in review audits. Apart from that, I understand that the system treats both in the same way. However, I personally prefer to limit spam flags for actual spam, cases where someone is trying to promote something, since that tells me as a moderator handling the flag what to look for. So yes, I would prefer you flag that as R/A instead of spam if only to help me get into the right frame of mind to evaluate the flag. But it really doesn't make that much of a difference, no.
By the way, I've written up a guide for which flag to use where on Ask Ubuntu's meta, and the directions there are also valid for here.