Don't give up! Flag, and comment (briefly) about why. It's been my experience that lots of storied that I believed were long past flaggable (80-90 upvotes) have been killed sometime after I flagged them.
Even if your flag doesn't kill the story, the conversation about why you flagged it will be valuable --- at the very least, more valuable than the off-topic story.
Even if your flag doesn't kill the story, the conversation about why you flagged it will be valuable --- at the very least, more valuable than the off-topic story.