In this instance, the prohibition would only be upon an act which is prohibited as a baseline in civilised society, but allowed under certain limited conditions where magic words are involved: when a 'doctor' who is 'licensed' is 'practicing medicine'. That is when somebody magically becomes permitted to subject a human body to damage with no legal intervention.

Simply rescind the right of the white-coat clergy to perform surgery on genitals for psychiatric, cosmetic or religious reasons. Done. Trannies instantly have to resort to mutilating themselves, which will soon stop after the first few performative 'drumpf made me do this!' garden-shears examples. With the stroke of a pen, you torpedo the money-making capacity of tranny medical industrial complex. Trannies losing their $$$ value as lifelong patients would deflate a lot of the social, institutional support of the ideology too.

Of course, trannies aren't the only or most powerful group who would protest...