For now it’s already possible to tag post. It’s also possible to mute or block users. So I’m unsure if we really need to add spam tag.
For unprivileged user I wouldn’t give right to mark whole topic as spam. As many times in topics there can contain some great and constructive posts and some post(s) which don’t belong there as they don’t add any value but sometimes even take some value of the whole topic.
Unfortunately there were some posts like that in the last 24 hours 
. But trolls are hungry individuals and they usually go elsewhere if we don’t feed them. They particularly like to feed from opposition 
. It’s a sweetest cream to them.