I'm not asking how to go about putting in place such a restriction. Just imagine that you could. Wouldn't that solve a bunch of issues, i.e. fraud, asset bubbles, consumer exploitation via high-interest debt, etc.?
Reasoning is that the incentive for profit maximization would be removed and replaced with maximization of value to the service recipient. Companies (and individuals) would get the financing that works best for them, as opposed to the financing that makes the most money for the banks.
Unless you're talking about splitting investment and commercial banking.. Look into the Glass-Steagall act.