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.

Are you talking about work-in-progress (WIP) limits? I've seen tools enforce this so you can't pull another story into the current list. Other tools let you pull but highlight the fact that you're going over the WIP limits by making the whole column eg. red. Or do you mean something else?

I've used LeanKit's Kanban board to enforce per-lane WIP limits. It works quite well and is, in my opinion, one of the most important aspects of using Kanban boards for software development.