Jilly,
We eventually taught our Border collie (and our other dogs <g>) the command "That'll do". It took persistance on our part, but they understand it means we are DONE with whatever activity they are currently involved in, whether that be fetching balls, tugging "stuffies" to pieces (our other mid-size dog is an Icelandic Sheepdog) or barking an alert (in the house) because the barn dog happens to be barking <sigh>. We don't allow house barking beyond a short, acceptable "alert" letting us know something or someone new is outside.
Having a Great Pyrenees out in the pasture is plenty of warning under most circumstances - she doesn't really need much in the way of back-up <g>.
Anyway, "That'll Do" USUALLY brings the activity of the two "herding dog" kids to an immediate end. At least they are smart and learn quickly, eh?
Patch, in particular, looks so cute when we give this command after a rousing bout of tennis ball chasing. When we've had more than enough and he brings the ball back AGAIN ... we say "Patch; that'll DO." He'll look hopeful for another few seconds, and when nothing happens, he'll give a big SIGH, drop the ball wherever he happens to be standing, and walks away, dejected. <hehehe>
Then he'll pick up a different toy and offer that to see if maybe, just maybe, the command was only for the ball <g>. Border collies are optimistic sorts, aren't they?