Banning thin models from the runway is wrong, in so many ways. It sends the message that all thin people are unhealthy, which is not the case. It is as wrong as it would be to ban fat models. The intent behind banning thin models is good, it is the execution that is problematic.
At the most, it would be okay to set a quota for thin models. Say, require that no more than 20% of the models be outside the typical BMI range (18.5 to 25), or something like that.