Back in the day, it was possible (tool issue) and likely (operator issue) that one would wind up in a situation in which something was deleted from all diagrams, yet remained in the model. Is that still possible and is the process for removing it to add it to a diagram so that one could then delete it from the model?
Yes. This is still possible. It is intentional and fundamental to how Rose works and most of the UML tools.
The metaphor I like to make is of a database and reports based on a database. In Rose (and most UML tools) there is a repository or database in which you store the model. One can construct a very valuable, informative, and well-engineering model without a single diagram. People often forget that UML stands for "Unified Modeling Language" not "Unified Drawing Language".
Anyway, the result is that the diagrams are loosely coupled to the repository/model and this is why I take pains to teach/coach the difference between delete from diagram and delete from model.
You can add something to a diagram and then take the "delete from model" action to remove it from that diagram as well as from the repository (and all of the other diagrams) but that isn't the way that I recommend.
The preferred way is to navigate the repository (that is in the left pane, uses a folder metaphor) and delete the item from the repository directly.
This is preferred for a couple of reasons.
It is, in general, a good habit to use the gestures on the diagram for working with the diagrams and the gestures on the repository for working on the repository. This is much more reliable - if only because it helps to reinforce in one's mind the scope/magnitude of the action you are taking and less likely one will make a mistake. Rose doesn't have a very good "undo" feature so not making mistakes is important.
Also, depending upon how your diagrams are setup, adding the element (e.g. a class) to the diagram so that it can be deleted may mess up a a working, valuable, diagram. You could get around this by using a "scratch" or temporary diagram but I still suggest working from the repository instead.