You could just start a new thread and then post 49 replies to yourself on the thread you just created. Instantly reaching 50 posts before a moderator spots you.

Posting automatically is not complex - the posting page is just a page with a form in with a few fields that need populating, this can be created automatically.

I doubt that the spammers actually post manually and just have purpose made code that can be used to post on the commonly used forums. As these spammers are well known and widely reported it is very unlikely that they manually post their rubbish.