Negative Remaining Labor Units or Alert for Impending Zero Remaining Labor Units

Once remaining labor units are spread among assigned resources and, for example, one of the resources exhausts the remainig that had been allocated, the remaining at the task level no longer is reduced by that resouces accumulating actual hours.  Can the remaining labor be set to display negative hours?  Can the task be set to prohibit future charges by resources via Progress Reporter?  or Can an alert be set to warn when remaining hours are reaching a critically low level?