In addition to bots & users panning for free crypto it's also possible that some folk simply lack the ability to express themselves in a meaningful way, or are just plain apathetic.
A comment screening script or bot similar to that proposed by @investigation may become necessary, one that assesses the length of the comment & checks its content against word/phrase lists, but I imagine this would need to run server-side.
Steemit has Cheetah searching for plagiarism, why not Chatterbox to detect comment submissions that are too short or generic and, potentially, warn the user on consequences for reputation if they choose to proceed with submission.