With the amount of Quickbooks SPAM hurtling it's way around the system, I find I can only flag so many of them as SPAM.
If the post has been deleted, does the system really, not hand back the flag to be reused? And if not, should it?
That's correct; the system does not let you reuse flags. Your consolation prize is that your flag was marked helpful, which counts towards increasing your future quota of flags. The Complete Rate-Limiting Guide has much more information; for this situation, search/scroll down to "Flagging".
I appreciate that there are good reasons for wanting flags to get rid of spam; I haven't thought through the mechanics of how a reuse mechanism would work overall. A quick search on Meta.SE didn't pull up anything pertinent, so if you're interested in such a change, I'd encourage you to think it through and post a feature-request on Meta. My only caution, coming from my personal limitations, is that the Stack Exchange codebase has grown (IMHO) fairly complex over the years and there are probably corner cases to think about.
All you need is a set of sockpuppets or voting ring with flagging privileges to make anything disappear from the site, simply by flagging it as spam. With additional spam flags, you can do a lot of damage this way.
As annoying as spam is, the more content that needs deleting, the more people need to be involved in deleting it... otherwise bad things will happen!
(We can't have nice things).