Easiest thing is for your friends using Messages to delete your contact and re-create it so your number is no longer attached to messages.
You should also remove you phone number as a link to your Apple ID, so you no longer receive SMS to your iPad. If you still have the iPhone you should deactivate it from Messages as well.
More info here:
http://forums.macrumors.com/showthread.php?t=1457873
NFtoBC
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