The best treatment and with a permanent result is by using a laser technology. this is a highly specific laser with wavelength of 1064 nm. It only target the melanin cell.
Multiple sessions (at least 4) are usually necessary. The number of session depends on energy level of treatment. Lower dose may needs 10 to 15 treatment sessions.
Normally after a full course of treatment, the result is permanent provided that
Baring these 2 conditions, there is no other exception to the rule.
Temporary tanning of the skin, called the post-inflammatory pigmentation, PIH in short, can complicate the treatment. However, this PIH usually settles over time by itself if not exposed to the sun. The risk of post-inflammatory pigmentation is increased if melasma is also present. The melasma will also need medical treatment.
Any bleaching cream will also help but never able to totally erase the problem. It can only lighten the lesions. The moment it stop the cream application, the lesions comes back in full force.
Microdermabrasion. This is not effective other than helping the skin absorption of various nutraceuticals.
Galvanic. Yes it can be useful to help delivers the lightening agents deep into the dermis.
IPL. It has no role to play at all. All Hori Nevus are deeply located into the dermis.