a.k.a. Rabbeinu Gershom Me’Or Hagolah (“our teacher Gershom the Light of the Exile”).
His unilateral edicts are pretty unique in Jewish tradition, but by the force of his stature, his will has prevailed over the centuries.
Takkanot of Rabbeinu Gershom [The Legal Adjustments (into Orthodox Jewish Law) of Gershom ben Judah]:
1) A prohibition on polygamy
2) A prohibition on summarily divorcing a woman against her will [but Divorce per se, after discussion and/or mediation remains thoroughly permissible, as prior]
3) A prohibition on reading the private mail (emails would be included – wink) of others
His bans are considered binding on (Ashkenazic, in particular) Jewry to this day.