IntelliJ IDEA suggests two ways of deleting elements from diagram:

To remove elements from view
  1. In the diagram, select one or more elements to be deleted.
  2. Press Delete.
To delete elements from project
  1. In the diagram, select the element to be deleted.
  2. Use the Safe Delete refactoring (Alt+Delete) to delete selected classes from project.

See Also

Procedures:

Reference:

Web Resources: