Marking a message as spam and blocking a have different use cases.
Spam is generic content that is sent to many people (usually advertising). The goal of marking something as spam is to let your system intelligently remove other identical (or similar) posts so that other members don't have to be presented with this. Gmail does an excellent job of this.
Blocking a user is about not wanting content from them anymore. Think of an ex-girlfriend or boyfriend who is constantly sending you messages, or a stalker. Blocking them prevents you from having to hear from them, but has no effect for the rest of the user base.
So, marking a message as spam, shouldn't automatically block a user. If someone wants to block all contact, they can block the user, but there are many cases where you may want to mark a message as spam, but still receive contact from the user.