The antonyms for "opens one's heart" can vary depending on the context. If we are talking about emotional vulnerability, some antonyms may include guarding one's heart, remaining closed off, or shutting down emotionally. Alternatively, if we are talking about physical barriers, antonyms could include closing, sealing, or obstructing. Keeping one's thoughts or emotions concealed could be seen as antithetical to opening one's heart. In some scenarios, opening one's heart may imply trust or connection, while refusing to open up could convey the opposite sentiment. Ultimately, the antonyms for "opens one's heart" should be understood in the unique context of the situation.