When expressing disagreement or a negative response, it's common to say "certainly not." However, there are many synonyms that can be used to convey the same sentiment. One alternative phrase is "absolutely not," which is often used to emphasize a resounding "no." Another option is "definitely not," which conveys a strong sense of certainty and leaves no room for ambiguity. "By no means" is another synonym that can be used to indicate a firm rejection. If looking for a more polite alternative, "I'm afraid not" or "regrettably not" can be used to express disagreement in a more tactful way.