If your ASL box is the only gateway to internet it's quite easy.
On ASL box : - in packet filter do not allow HTTP/FTP/HTTPS trafic from internal to outside (on rule like "internal->any,http (or https or htp),allow" or "Internal->any,any,allow") - activate HTTP proxy in standard or transparent mode, just keeping internal as allowed network in proxy settings.
NB:If you choose Standard mode, you will also have to change proxy parameter of the web browsers in your internal network. If you choose Tranparent mode only HTTP trafic is handle by the ASL proxy, so you may have to add specific packet filter rules on ASL for FTP and HTTPS trafic if needed.
is it a windows network we are talking about? If so, is there a domain-controller? You can solve it then wit de GPO in the AD and fix it, so that the users can´t change the proxy-settings. p.s.: this only works with IE.
is it a windows network we are talking about? If so, is there a domain-controller? You can solve it then wit de GPO in the AD and fix it, so that the users can´t change the proxy-settings. p.s.: this only works with IE.