So there have been posts that I have flagged that were both spam and off-topic.
People should be able to flag for multiple reasons, such as: spam and off-topic, spam and innapropriate, off-topic and innapropriate, and so on (even all three!).
I would go with what @byc14 says, but if you have a strong use case for this that “Something Else” cannot fulfill, you should file a request @ discourse.meta.org since that is the forum software that this forum uses.
There are already feature requests on meta discourse about custom flags and checkboxes for flags, however it’s been 2-4 years and they have not given an update about this.