You can do this globally in each area's settings, or you can use the PLAIN BB-Code to prevent the smilie conversion.
Unlike vBulletin, XenForo does not have a built-in function to disable smilies for individual entire posts. Adding the option on the VW side only would be the same level of inconsistency as the "automatically parse links" option which was finally removed from VW4+XenForo in 4.0.17.