I don't know which astrologer first said that conjunctions cannot be prohibited in horary. I would not be surprised to learn that it was a dead astrologer, and living ones simply keep repeating whatever the dead one wrote—as usual, without any corroboration in practice.
Conjunctions, like any other aspect, can be prohibited. Prohibitions must be interpreted according to the context and must take into consideration dispositorship, receptions, accidental dignity and the meaning of the planet that is supposedly prohibiting. Only after all those things are taken into consideration we can make decently acceptable determinations.
