You can't. Whether or not you are sent TNEF files depends entirely on the settings that the person that sends the message is using. You can contact the person in question, and ask them to change their settings.
In general, two things must be true for a TNEF fle to be sent. Firstly, the sending e-mail program must believe that it is sending to a server that understands TNEF files. This typically occurs when, in Outlook, the properties of your e-mail address are set to "Send using Outlook rich text format" Secondly, the message format iself must be RTF or HTML. Note that the address property of "Outlook rich text format" is a propery of the e-mail address, NOT the message. The person that is sending the e-mail should ensure that both the "Send using..." property of your e-mail address and the format of the e-mail are BOTH non-RTF