Right-click the corresponding object and select Remove.
If you want to remove several object, mark all of them and then proceed as described above.
Using the Delete key you can delete all highlighted elements, i.e. also tracks, nodes, etc. Before this command is executed you will be queried if you really want to delete the listed elements. Tip: Hit Ctrl + D to clear all highlights. Following this highlight all the elements that you want to delete.
See also Marking objects
This function removes objects from the timeline. The media files, however, remain in the Media Pool. Please remember, however, that any editing of the objects, e.g. cuts, dissolve ramps, keyframes, etc. is lost.