When trying to delete an obsolete view the system indicates that there are relations left thus prohibiting this delete. See attached screen print.
The system complains about a relation with an 'Application Account'. For some 'default view'; however I don't even see how one can specify a default view for an account. I have even removed all the roles for the application account and the system will still not allow me to delete the view.
Also there appears to be a relation with something called 'Item'. I am totally lost on that one; I have no idea what that is supposed to mean.
Users can set the view they want as their default in their client (as long as the view is related to their role of course).
Now, if a user set that view to be its default view, then this was probably stored in its cache data, so maybe ask that user to login to SD and to use any other view as its default (View->Current view-> Define Views) then to logout and login.