Please use the Question Tags to state the name and full version number of the Story Format you are using, as answers can vary based on that information.
You can place CSS like the following with your Story Stylesheet area to hide the Harlowe left side-bar (which contains the Undo and Redo links)
tw-sidebar {
display: none;
}
note: I suggest hiding the whole left side-bar instead of the individual links it bar serves no other purpose.