Grease is a ridiculously simple addon that lets you unsticky various chat types to your liking. By default, it reverts the chat window to pre-3.3.5 functionality, where whispers and numbered channels must be specified each time you wish to use them.
Type /grease to bring up the options window. Checked chat types are sticky. (Pressing Enter will use that chat type if it was the last one used.) Unchecked chat types must be specified each time. (They're not sticky; they've been Greased!) It's that simple!