font.rb

class Font < ApplicationRecord
    has_many :styles_fonts
    has_many :styles, through: :styles_fonts
end

style.rb

class Style < ApplicationRecord
    has_many :styles_fonts, -> { distinct }, dependent: :destroy
    has_many :fonts, -> { distinct }, through: :styles_fonts
    accepts_nested_attributes_for :fonts
end

styles_controller.rb

class Admin::StylesController < Admin::BaseController
   def delete_font
    @style = Style.find(params[:id])
    @style.fonts.delete params[:font_id] # ActiveRecord::ActiveRecordError (delete_all doesn't support distinct):
  end 
end