No, I've never seen it enforced by tools. Physical (index cards) kanban boards have an implicit space limit though, and this is one of the too-seldom-acknowledged reasons why they work as well as they do. Unfortunately the software clones of physical kanban boards copied the unnecessary part (visual appearance of index cards) and not the necessary part (limited space at each phase).
Rally lets you set card limits in each column, though in practice that always seemed too easy to change (just add a couple more to the limit; just "temporarily" turn off the limit) to be entirely beneficial.
