Receiving someone else's text messages?

I deactivated the Verizon service on my iPhone 4 a few months ago and switched to a new AT&T phone (new phone number). I have been using my iPhone 4 as a media player and for browsing e-mail/internet on wi-fi. I'm still able to send/receive text messages on my deactivated iPhone using the old phone number while on wi-fi.

Recently, my iPhone has been getting text messages from random phone numbers. The text messages are usually mid-conversation in nature, stuff like, "ok yeah let's do that". I've responded by saying that they have the wrong phone number, but they'll just take it as a joke and continue their conversations.

I've made sure that my iTunes account and my e-mail address are the only things logged into iMessage.

Where should I go from here? Thanks in advance for any help.