The [delete] movements dialog appears very similar to the [Open] movement. The main difference in the display is the ability to choose more than one movement. Only movements next to each other in the list can be in your multiple choice list.
One way to limit your choice of movements for deletion is to change the selection rules. For example you might choose "Mitchells" and then click [select all] button. Another way to choose a selection is to the click the first movement for your list and hold the [shift] key down and click the last required. All intervening movement will be highlighted and in the chosen list and will be deleted once the [delete] button is clicked. Using the shift key in conjunction with the down and up arrow keys will achieve a similar result.
Any movements deleted from the current library will be lost and cannot be recovered. One way to keep copies of the movements is to make a new library and copy all the movements to that library before deletion. To backup a complete library before deleting any movements use the [Export library] option.