If your concern is that when these smilies turn into "J" on a plain text device, like a Blackberry, there's another option besides disabling it entirely, which is changing it to replace with the Unicode smiley character, 0x263A: ☺
A "quick" way to do this is to open the "Insert symbol" dialog in Outlook, type 263A into the Character code box (bottom right), then click the AutoCorrect... button in the bottom left to create a new autocorrect rule for your preferred smiley to the text-safe Unicode equivalent.
Here's a Lifehacker link with more detail & more difficult approach to get to the autocorrect entry.