Darkness in the Sunshine State
Gus has banned Wendell from the store on several occasions, for everything from loitering to harassing customers to, most egregiously, rearranging the books on the shelves. But for some reason, Gus always relents and gives him another chance. Until the next time he bans him.
Wendell is supernatural in some as-yet undetermined fashion, something Gus subconsciously picks up on, and which is probably the reason he keeps lifting the ban. Why Wendell has chosen to focus on the store remains a mystery.