The phrase "lets upon" has multiple antonyms depending on the intended meaning. If "lets upon" is used to mean "grant permission," some antonyms could be "forbids," "denies," or "refuses." On the other hand, if "lets upon" is used to mean "puts forth," "suggests" or "introduces," some antonyms could be "withdraws," "retracts," or "withholds." Lastly, if "lets upon" is used to mean "explains" or "reveals," some antonyms could be "conceals," "hides," or "obscures." It's important to determine the intended meaning of "lets upon" to come up with the appropriate antonym in context.