I actually think that the "low quality" flag makes the most sense. I just reviewed and voted to delete a post marked low quality, and it was easy and fun. It's also good in that I think many people were hesitant to flag things as spam that weren't traditional spam, but there should be no disagreement about voting to delete things that are "low quality".
UPDATE : To give folks an opportunity to test-drive the "low-quality" system, I have flagged the following bit of spam as low-quality : http://mathoverflow.net/a/134724/317