"Illegitimatize" is a rare and complex word that means to declare something illegitimate or illegal. When looking for synonyms, it's important to consider the context in which the word will be used. Some potential alternatives could include disqualify, discredit, invalidate, annul, cancel, void, or nullify. These options all share the sense of negating or destructive power, as well as serving to legally or morally undermine something's credibility or legitimacy. Other possibilities could include delegitimize, depose, impeach, or repudiate, depending on the specific situation or subject at hand. Choosing the most appropriate synonym will ultimately depend on the intended tone and meaning of the message being conveyed.