OK, thank you for your replies.
@Barcode_link, I understand your point is that there are too many posts (and new threads) that do not violate forum rules but are "spammy" and trivial, and the users posting them would not be punished even under the proposed rule (report->deleted post->signature removed) because the moderators could not delete them, because there is no rule with respect to this kind of posts. Is that what you basically mean?
In this case, I would propose a new forum rule allowing moderators explicitly to remove these "spammy" posts (for example, the rule could be that "trivial answers which already were discussed at the topic" could be removed).
If the whole thread is being sent to the trashcan by a moderator, every users posts on those threads will be remove too, and I believe we will see another flood of threads on the meta section again created by newbies and Jr. Member users asking why their profile signature is gone, another hot debate will surely starts, as they will try to explain that their posts is not spam, they should not deserve to lose their signature space.
But couldn't that be solved technically? I don't know the internals of the SMF software, but I think a "report" should be possible to be registered as an action by the software, and a "post removal" separately. So all what had to be done would be to only remove the signature if the post itself was both "reported" and "deleted", and so the other users posting in these threads would not be affected at all.