Well, there is the plain tag. If you are talking about YOUR posts. Code: This is a line without the plain tag :) This is a line with the plain tag [plain]:)[/plain] This is a line without the plain tag This is a line with the plain tag :)
TheWindBringeth already answered the question and gave the solution (using the PLAIN tags ), so the following may be not needed, but anyway: there is a few months old thread in which it was discussed too: smilies - how to disable? https://www.wilderssecurity.com/threads/smilies-how-to-disable.362568/
I found the other thread using a Web search but not via using the Board search as I generally do that before duping something that already exists. All is under control - it's a XenForo thing and not user induced.