The phrase "be a reproach to" can be substituted with various synonyms depending on the context. One possible synonym is "bring shame to," which implies causing embarrassment or dishonor. Another synonym is "cast a negative light on," which suggests a negative perception or impression being formed. "Reflect poorly on" is also a possible alternative, emphasizing how the actions or behavior of the subject have a negative impact on their reputation or image. Other possible synonyms include "undermine," "denigrate," "tarnish," and "discredit," all of which convey a negative connotation and indicate a loss of respect or esteem.