Sometimes Second Life's asset servers get into trouble. When that happens, you can't do much. Teleports are limited, you can't open notecards or scripts, you can't import textures or photographs, you can't rez objects from inventory, and you can't delete objects on the ground. You zap them, and a minute later they're back, along with a "object returned to the simulator" message. Frustrating!
There's a way to get rid of objects, however. Chey discovered it last night.
Just turn them temporary. After a minute or so they will disappear. Permanently.
To turn an object temporary, just edit it and select the Object tab. On the left side of the edit box, just below the horizontal row of tabs, check the box that reads Temporary. End of problem.
Be careful with this! If you turn your valued living room set temp, it will disappear permanently, with no warning. You won't find that couch and its cushions in your trash, either. It will just be gone.
It's good as a rule to set ONLY copyable objects to temp. But remember, when you just can't seem to get rid of a prim or linked set of prims, Temp will do the job.