Emmanuel,
unfortunatly, i have to report the very same bug here (with 3.0). "Sometimes" i simply can't delete (remove) a furniture item from the current plan. Sh3d "responses" like i actually have deleted something, but after a second i can dblclick the object again - it never disappeared.
It is always a big project compared to my Macs memory, and i solve that by: saving plan, quit sh3d, reload sh3d, reload plan, THEN delete the item again (and save again).
it's a memory/undo issue, imho.
The undo is stacked up and never being freed until quit (on Mac only, i'm afraid).
it is not that bad, only anoying. maybe already fixed with 3.1?
Also, it seems to be mostly impossible to group lights and door/window objects.

macfrog