With a proper grammar, you can require the "subject" field to be one of several valid entity names. In the prompt, you would tell the LLM what the valid entity names are, which room each entity is in, and a brief description of each entity. Then it would be able to infer which entity you meant if there is one that reasonably matches your request.

If you're speaking through the kitchen microphone (which should be provided as context in the LLM prompt as well) and there are no controllable lights in that room, you could leave room in the grammar for the LLM to respond with a clarifying question or an error, so it isn't forced to choose an entity at random.