You can delete lifelines to remove the visual representation of
an instance of a classifier.
You must have a sequence diagram open that contains lifelines.
To delete a lifeline, in the diagram editor, right-click a lifeline;
then click
Delete from Model.
All send and receive
messages that originate from the deleted lifeline and all interaction fragments
that cover the lifeline are deleted. The combined fragments and interaction
uses that cover other lifelines are not deleted.