It is possible to have trademark rights in a particular depiction of the grim reaper, even for costume/Halloween related services. That doesn't mean that trademark rights in such an image, even if registered, prohibit uses of all depictions of the grim reaper by competitors. To the contrary, only confusingly similar uses would be prohibited.
Jeff,
Let's say arguendo the situation is the letter sender has a registration where the Description of Mark is recited in excruciating detail with respect to the logo and store name (the colors used, the posture and positioning of the reaper, existence of and positioning of ancillary items in the logo).
How does the verbal Description play into likelihood of confusion? (Or does it at all?)
Along a different tack, if the letter sender's mark is so different from another's use that you can presume no good faith basis exists, are there any sort of penalties that may be assessed for what is essentially trying to use federal law as a bullying stick?