I am suspecting there is something you are leaving out. A proxy can run in a number of ways. You can put it on a single IP for both the inside and outside, you can run it as sorta a router with a different inside and outside and you can run it transparent sorta like a switch.
If your goal is to force users to use the proxy and you have no way to prevent them from changing the proxy settings then I would run it in transparent mode. Since the users would not get the proxy settings they is nothing they can change to attempt to bypass the proxy. If you have a cisco router you can also use WCCP to force traffic though a proxy server