If a runner is being held on base by a fielder other than the pitcher, and the fielder won't throw the ball back to pitcher for fear of the runner taking another base on the throw, how should this stalemate be resolved? May the Ref call Time to end the play? Is it considered "stealing" per Rule 10.03 to run on a throw back to pitcher following a stalemate?

When such a stalemate occurs between a fielder an runner, and no other runners are attempting to (or are able to) advance, then the play is essentially over. At this point, it is good form for a Referee to call a time out (see Rule 3.03a) and move the game along, but even if time is not called, a Referee may send a runner back who attempts to "steal" a base on the throw back to pitcher that ends the stalemate.