title: Stop corutine after reacting to a message
body: <p>I am making a discord bot which sends dm's and the user needs to react to the message to interact with the bot. </p>
  
  <p>First of all you call the bot command with the prefix and the command name. After that, the bot will dm the user tagged in the message. The bot will request you to react with an emoji, and after that it will execute some stuff. The problem is that I don't know how to stop the bot from executing that stuff after the first reaction. Lets say the user reacts with :thumbsup: --> the bot makes some stuff, but if you then react with :thumbsdown: (or delete the :thumbsup: and then react again) it will keep making that stuff, I dont want that to happen. </p>
  
  <pre><code>@client.command()
  async def yesorno(ctx, member: discord.Member, *, content):
      mens = ctx.message.mentions
      mens.append(ctx.author)
      if len(mens) != 2:
          await ctx.send('One user only')
      else:
          # await ctx.author.send(content)
          await member.send(content + ', react either with , or ')
  
      @client.event
      async def on_reaction_add(reaction, user):
          channel = reaction.message.channel
          if channel == user.dm_channel:
              if str(reaction.emoji) == '':
                  await ctx.send(user.mention + ' accepted')
                  return
              elif str(reaction.emoji) == '':
                  await ctx.send(user.mention + ' declined')
                  return
      return
  </code></pre>