Update: The "spam" closing reason has recently been removed.
As far as I know, a user gets a 100 rep penalty when one of their posts is flagged as spam often enough. I just noticed that this does not happen when the question gets closed as spam.
I assume this difference in behaviour is intended, and thus it suggests there's supposed to be a difference between flagging as spam and voting to close as spam. So what should be done in what case?
When should I only vote to close as spam, and when should I also flag as spam (the third possibility, flagging but not closing, probably doesn't make sense)?