If she is indeed a small part devil, as the author text speculated/implied, then the only thing that matters is the wording of the contract and the mutual signatures, the physical paper that is signed as a formal agreement. Since she's not an actual devil, the contract itself is not maintained by some supernatural power, and can be made void by anyone with authority to do so, per the contract's own rules, a fact HW has exploited by making herself the LLC's manager, thus leaving her the only one in the LLC with the power to alter the contract. But since the contract in this case is for a registered LLC in the normal legal system, anyone with the power to suspend an LLC has such authority. Once the contract is void - say, by means of a big red stamp across it - Hench Wench loses the powers that come from it.
That's how I would explain it, at least.