"Letted next to" is not a valid phrase or expression, as "letted" is not a proper verb form in English. However, if we consider the word "let" on its own, its antonyms might include words such as "prevent," "hold," "disallow," "forbid," "prohibit," or "stop." These words convey the opposite meaning of "let," which usually implies allowing or permitting something to happen. Antonyms can be useful in expanding our vocabulary and expressing different shades of meaning in our speech or writing. Using antonyms can also help us to clarify our ideas and intentions.