Help with cache-proxies

Hi to all, i have a big problem, i use a phpBB forum in my page, here in
Spain the ISP's use a cache proxie system.

And when change anything in my forum such as image or new functionality, my
users can not view the forum.

Yesterday i include a visual system confirmation to register, and today my
users can not view the forum.

This is the header directives of the header file of the forum

if (!empty($_SERVER['SERVER_SOFTWARE']) && strstr($_SERVER
['SERVER_SOFTWARE'], 'Apache/2'))
{
header ('Cache-Control: no-cache, pre-check=0, post-check=0');
echo "Apache 2";
}
else
{
header ('Cache-Control: private, pre-check=0, post-check=0,
max-age=0');
echo "Apache";
}
header ('Expires: 1');
header ('Pragma: no-cache');

Any one can help me

Excuse my broken english





Un saludo, Danny