It's not only the victim that can notice this, but anyone that saw the original post and noticed that it was deleted as spam/offensive.
There are differences between the malicious spam flags you describe and real ones that could be used to protect against this kind of abuse:
- real spam flags tend to be cast against very new posts, not ones a year old or so
- the user of the spam post almost always has only 1 rep and no positive participation
It might work to simply require moderator action on a spam flag instead of acting automatically if the target user has a significant amount of rep. Those cases should be very rare, and either false positives, or self-promotion cases that would benefit from moderator involvement anyway.
Offensive flags are a bit different. I don't think they cause IP-based measures, but I'm not entirely sure. But I think a strategy similar to the spam flag one could work, as several valid offensive flag against an established user would indicate a situation where moderators might want to be aware even if the post is automatically deleted to enact further consequences.
Alternatively, the flags could be applied automatically in every case just like now, but trigger community-cast flags to make the moderators aware and either confirm everything is okay, or deal with the situation.