destroy_one_attachment.rb

# Model
class Profile < ApplicationRecord
  has_one_attached :avatar
  accepts_nested_attributes_for :avatar_attachment, allow_destroy: true
end

# Controller
class ProfilesController < ApplicationController
  before_action :set_profile, only: [:show, :edit, :update, :destroy]

  # [...]

  # PATCH/PUT /profiles/1
  # PATCH/PUT /profiles/1.json
  def update
    respond_to do |format|
      if @profile.update(profile_params)
        format.html { redirect_to @profile, notice: 'Profile was successfully updated.' }
        format.json { render :show, status: :ok, location: @profile }
      else
        format.html { render :edit }
        format.json { render json: @profile.errors, status: :unprocessable_entity }
      end
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_profile
      @profile = Profile.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def profile_params
      params.require(:profile).permit(
        :last_name, :first_name,
        :avatar,
        avatar_attachment_attributes: [:id, :_destroy]
      )
    end
end

# View
<p id="notice"><%= notice %></p>
<p>
  <strong>Avatar:</strong><br />
  <% if @profile.avatar.attached? %>
    <%= form_for @profile do |f| %>
      <%= f.fields_for :avatar_attachment_attributes do |avatar_form| %>
	<%= avatar_form.hidden_field :id, value: @profile.avatar_attachment.id %>
      	<%= avatar_form.hidden_field :_destroy, value: true %>
      <% end %>
      <%= f.submit "Delete avatar" %>
    <% end %>
  <% end %>
</p>