There are several reasons to get posts deleted. It is not always neccessary, that posts are spam. There are a few examples, who can delete posts or how spam is detected.
- There are moderators, who take care about spam posts
- Also members can report spam to moderators
- There are spam bots (don't know how they work)
- Members can open self moderated threads and delete any posts they wish
- Sometimes threads are deleted and all posts in them