There is a rule against it.
You're allowed to field any of your 25 man squad and it not be considered "weakened". However, there is also:
The "Utmost Good Faith" Rule (Rule B.13)
- The Obligation: All member clubs must behave toward each other and the League with the utmost good faith.
- The Application: This overarching rule means teams must compete honestly. Intentionally "throwing" a game to hurt a rival's title chances, help a friendly club avoid relegation, or secure a better draft/positional metric is a direct breach of this contractual commitment.
So can we expect West Ham to make an appeal against Villa? I doubt it.