no_double_submit_jquery.coffee

  $('#quick_entry form').live 'submit', (event) ->
    event.preventDefault()  # just moved up!
    event.stopPropagation() # just moved up!
    return false if @beenSubmitted
    @beenSubmitted = true
    clearNotice()
    $.ajax({
      type: 'POST'
      url: @action
      dataType: 'json'
      data: $(@).serialize()
      success: (response) ->
        setNotice(response.bill)
        resetAndFocusForm()
      error: (response) ->
        if response.status == 422
          setErrorFields JSON.parse(response.responseText)['error_fields']
        else
          alert("An error occurred!")
      complete: =>
        @beenSubmitted = false
    })

As other commenters have suggested: Just put false at the end rather than putting event.preventDefault() and event.stopPropagation() at the beginning.

You said you were concerned that if an exception is thrown, the return false won't be reached. But unless clearNotice might throw an exception, there's no chance of that; any error that occurs in $.ajax and the callbacks you give it will happen asynchronously—after the function has already returned.