You'd have to edit dozens and dozens of templates (my best guess is close to 100 templates). Fractional is what the system is based on, so its the primary display (and in some places, the ONLY display). Decimal Odds are a secondary display that you can enable and when enabled, you can choose whether to display them in European format or Non EU format.
All the sportsbook templates are prefixed (nflj_sportsbook_ or alert_sportsbook_ or news_feed_item_sportsbook_ or report_sportsbook_) (you can filter by those prefixes). be careful not to edit any INPUTS or you will BREAK the save actions.
I'll look into the feasibility of having an option to disable them in the next MAJOR version (which will be for XF 2.0 series).