I have found a number of comments consisting entirely of question marks, and a few consisting entirely of underscores, or tildes, and a few other variants.
These comments should be auto deleted when they are flagged.
Currently, comments consisting entirely of periods are auto deleted, so this would be consistent with that. I have yet to encounter a legitimate comment that consists entirely of a single character.
Perhaps simpler but also effective: Comments consisting entirely of non-alphanumeric characters (even a mix of different types, e.g. "?!?!?!?!?!") could be added to the auto delete filter.