Proxy accepts old NT passwords. Why?

We’re running Windows NT on our servers. Our desktop computers run Windows 95 or Windows 98. We’re using Proxy Server 2.0 on one of our servers, and I’ve set it for Windows NT Challenge Response. This was working great until recently when we implemented a network-wide policy change, forcing our users to change their network password. However, Proxy is still accepting our old network passwords. Can I force Proxy to accept only the current passwords, and if so, how?