Hello, is there any way to force user to pass throught a proxy server using a L3 switch?
This wiki link lists some vendors that support it
It does not list ISA but you could always ask the vendor.
This is what WCCP is designed for. You can read about it here:
A much more messy approach (and probably unscalable) would be to use policy routing on your switch to divert all web traffic to your proxy server (if your switch supports PBR)