You can attach an event handler to the keydown or keypress event for the contentEditable field and cancel the event if the keycode identifies itself as enter (or shift+enter).
This will disable enter/shift+enter completely when focus is in the contentEditable field.
If using jQuery, something like:
$("#idContentEditable").keypress(function(e){ return e.which != 13; });
...which will return false and cancel the keypress event on enter.
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