There is a spam bot detection and it detects all those threads pretty reliably, from what I can tell. It just sometimes takes a few seconds or minutes before it marks such posts as spam. I don't know why, I have no insight into the technical programming of the forum software, but there is a system and it kind of works.
Note that those scripts have to be careful in what they detect as spam to avoid false positives. For example, if simply every post with "love problems solution" would be deleted and the poster would be banned, you'd be a victim by now ;-)