you could apply Alpha numeric server validation to the text area, this will allow only numbers and letters to be added and will fail on the : and // characters in the URL.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.