[IR SS4] Cannot detach or delete the ThreeDSpace - why not?

Somehow, one of my sheets has developed a 3D model - probably in response to some kind of design data interaction (I haven't worked on this project in months). It's empty, there are no graphic elements in it at all. I'd like to get rid of it. InRoads won't let me. I get the message "You cannot Detach or Delete the ThreeDSpace of the Active model. The operation is cancelled."

Why...? I can appreciate that this is probably important for Design models in design drawings. I'm in a sheet model, with no design model in the DGN at all. Heck - the "ThreeDSpace" is a sheet model (somehow).
Why does the operation cancel itself? Why not just have a warning, telling me I'm about to detach the ThreeDSpace, am I sure? Why do I find myself blocked from handling my models as I see fit, and why is the software not letting me do my job?

This ThreeDSpace model keeps getting in my way...It shows up any time I create a PSET, and I have to remember to delete it. It wants to be the "default" model whenever I need to attach this file to any other drawing. I've had engineers ask me what it's for, thinking they need to do something with it. It's serving no good purpose and creating extra work.

Is there ANY way to detach, delete or deprecate this unwanted model so that it no longer exists in my plan set?
Thank you.

MaryB