Hi!
As Kazy said, the del button on your keyboard will do the trick!
Here are some helpful buttons for you
Useful keyboard and mouse shortcutsMouse shortcuts:
Right mouse click drag / mouse wheen click drag - Pan.
Wheel up - zoom in.
Wheel down - zoom out.
Left mouse click drag over selection - move selected objects (can be combinated with WSAD screen control).
Left mouse click drag over edge of selected resizable object - resize selected object (can be combinated with WSAD screen control).
Left mouse click drag - selection region drag (can be combinated with WSAD screen control).
Ctrl + Left mouse click drag - add objects to selection by dragging selection region (can be combinated with WSAD screen control).
Alt + Left mouse click drag - substract objects from selection by dragging selection region (can be combinated with WSAD screen control).
Shift + Left mouse click drag over selection - select object behind selected object.
Keyboard shortcuts:
Ctrl + Z - undo.
Ctrl + Y - redo.
Delete / Backspace - delete selected objects.
H - mirrors selected objects horizontally.
V - mirrors selected objects vertically.
W/S/A/D - move screen in 4 directions (can be mentioned as WSAD screen control).
Shift + W/S/A/D - move screen in 4 directions faster.
Q - toggle additonal panels auto-hide.
Ctrl + C - copy selected objects to "Primary" clipboard.
Ctrl + V - paste objects from "Primary" clipboard.
Ctrl + 1 - copy selected objects to "Secondary 1" clipboard.
Ctrl + 2 - copy selected objects to "Secondary 2" clipboard.
Ctrl + 3 - copy selected objects to "Secondary 3" clipboard.
Ctrl + 4 - copy selected objects to "Secondary 4" clipboard.
Ctrl + 5 - copy selected objects to "Secondary 5" clipboard.
Ctrl + 6 - copy selected objects to "Secondary 6" clipboard.
Ctrl + 7 - copy selected objects to "Secondary 7" clipboard.
Ctrl + 8 - copy selected objects to "Secondary 8" clipboard.
Ctrl + 9 - copy selected objects to "Secondary 9" clipboard.
Ctrl + 0 - copy selected objects to "Secondary 0" clipboard.
1 - paste objects from "Secondary 1" clipboard.
2 - paste objects from "Secondary 2" clipboard.
3 - paste objects from "Secondary 3" clipboard.
4 - paste objects from "Secondary 4" clipboard.
5 - paste objects from "Secondary 5" clipboard.
6 - paste objects from "Secondary 6" clipboard.
7 - paste objects from "Secondary 7" clipboard.
8 - paste objects from "Secondary 8" clipboard.
9 - paste objects from "Secondary 9" clipboard.
0 - paste objects from "Secondary 0" clipboard.
http://www.plazmaburst2.com/level_editor_manual/