*sigh* ISA... InSecurity and Annihilation. The low-performance dog proxy of choice for system administrators who know nothing about computers or how they work.
If they're stuffing you through a proxy, transparent or otherwise, titting around with host files is futile. Addressing the site by IP address would be the best strategy, and you might also consider using https (instead of http) as ISA can't proxy encrypted traffic. PPRuNe supports https, which is nice.