Refresh this forum is an old feature (which I think have been stripped out but admin field is there still), this used to refresh the forum via AJAX every 30secs/ 1min etc.

One way you can stop SilverStripe from caching is to put this in your mysite/_config.php file HTTP::set_cache_age(0);. Or if you just want the forum holder view to not cache just try put that in forum/code/ForumHolder.php inside the

function init() {
// put it here
...
}

And see if that makes a difference

I think even if you put it in forum/_config.php it will still include it site wide. You could try (it the set_cache_age works) in your own Page.php file - in the function init() { if($this->ClassName == 'ForumHolder') { HTTP::set_cache_age(0); } ..