"ROT BS", an acronym for rubbish or nonsense, can be substituted with various synonyms to express disbelief or disagreement effectively. One substitute is "balderdash", which refers to senseless talk or writing. Another alternative is "hogwash", meaning absurd or foolish ideas or statements. Additionally, the phrase "drivel" can be used to denote nonsensical or trivial writing or speech. If you want to emphasize the lack of truth or validity, "poppycock" can be utilized, which implies utter nonsense or foolishness. All of these synonyms offer colorful ways to convey one's skepticism or dismissal towards foolish or baseless information.