In Sites and services just make sure the appropriate DC's are assigned to the appropriate sites for their physical location. That will ensure the most efficient replication. If you do go ahead and remove a DC you shouldn't need to worry about the clients. There may be a short period where the DC remains in their "memory" but once they find they can't talk to it they will move on to the remaining DC's quite quickly.

The clients will use whatever DC they damned well please as far as I can tell but they usually will find the closest one on their own. I would just check the clients for the appropriate network setup and that they are properly connected to the domain.