GSAS-II does not provide a restraint to keep \(\rm U_{iso}\) values positive or for that matter from refining to values that are too large (which is worse). This is not an oversight. Implausible \(\rm U_{iso}\) values are telling you something about your data and/or your model that you do not want hidden by a restraint. Either the data are not sensitive to the \(\rm U_{iso}\) values that refine to unbelievable numbers. This should not be hidden by a restraint. In that case the values should be set and not refined, or grouped so that the value has more leverage. Alternatively, the data are indeed sensitive to those \(\rm U_{iso}\) values, in which case your data is sending you a subtle message that there is something wrong with your model. Listen to that.