Stop talking, Please!

Have you ever had a co-worker who will not stop talking! It does not matter what anyone else has to say...they keep this running commentary on anything and everything, no matter how how busy everyone else is...they really can't seem to stop themselves. Trying to ignore them and not responding to them doesn't work. Even telling them you are really needing to focus on what you are doing and can't talk right now, doesn't seem to make an impression. (Well maybe for about five minutes, then they start in again.) It is hard when you have known them for years and would love to hear what they have to say, if only there were time to spend listening; but, when you are on the clock for your employer, it is difficult to balance the two.I try to be sympathetic to people when they need to get things off their chest. Also I know excessive talking can be a sign of nervousness. Maybe a way to get attention?? I'm really not sure, but I have exhausted the ways I know to politely say "Stop talking, Please! I can't think straight with this chatter going on..." I am worn to a frazzle (my other co-workers are as well) and need any suggestions. Any out there?? Thanks...