The best way to do it is to flag the post itself that was being vandalized and use the "Other" reason. In the box, explain carefully and briefly what the problem was: That user X was trying to edit spam in. Be careful that you are sure not to suggest the original poster was trying to edit in spam, but that it was someone else. You can include a link to that other user so they can check it out.
You can do this with just one flag by just telling the mod in the "Other" flag that they have been doing this multiple times; the mod will be able to look in the user's edit history to verify.
You didn't ask about this or suggest you were doing it this way, but for posterity's sake: don't use the "Spam" flag on the post for this sort of thing. That does not allow you to indicate any special information, and the mod will believe you were trying to say the original poster had posted spam, which is not correct, and would be declined.