Several questions about hotkeys and removing lines

Select the row(s) you want to remove (by clicking in the far right column of each row, that shows a black dot for each), then hit Cmd+D (Edit/Delete Rows).

You can select more than one row by clicking the first row, then shift-clicking the last one, or Cmd+click (Ctrl+click in Windows) to select just one row without affecting the others. All of this clicking is in the far left edge.

If you want to delete an entire position, click the black-dot column header, which will select all the lights currently on the worksheet.

If this is new to you, try working your way through the Lightwright tutorial, it's under the Help menu.