Do your users want Discourse to display :-) instead of 
?
In that case, if you are an admin, you can already disable smileys for the whole server (site-wide disabling).
if you want a user-wide disabling, could you precise if you want for a given user to have smileys disabled for all posts he reads, or only for his own posts?
In the 1st case, be careful, I don’t think Discourse would translate :slight_smile: in :-) when it has been written by an user who didn’t disable smileys.
For me, I asked for disabling only emoji auto-complete, because it bothers me only while writing posts, seeing :slight_smile: instead of :-). But it doesn’t bother me to see either in the preview pane or while reading posts.