The smileys are part of the wiki style renderer. In order to remove it across the board, you will need to configure the text area fields in question (probably the 'Description' field, but maybe other text areas you have set up too) to use the default text renderer. See more details at https://confluence.atlassian.com/display/JIRA/Configuring+Renderers
Alternatively, if you just need to disable it for a particular bit of text but want to keep the wiki style renderer enabled, you can enclose your text in {noformat} tags or you can escape the part of the text that is being interpreted as a smiley by using a backslash, e.g. ":\)" will render as ":)"
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