Twitter does not allow users to send direct messages to people who do not follow you, though that may act as a deterrent to stop spamming, direct messages are much more better than sending a message via a reply.
Tech Spikes has a nice hack that will allow you to send a direct message to people who do not follow you in twitter.
To send a DM to a non-follower in Twitter, you have to derive their DM URL to create a DM message. Here’s the syntax
http://twitter.com/direct_messages/create/twitterusername
Replace the twitter user name with the twitter id of the person to whom you wish to send the DM and you can send a direct message to a user without them following you.