The above will manually clear out the windows DNS cache. Note this doesn't block it, but rather simply limits what it can find out. I'm not sure it would be possible to block this functionality entirely without also breaking DNS, or otherwise causing the Anti-cheat tool to detect something odd.
By default, the Windows DNS cache doesn't flush itself, with entries lasting for 24 hours or so. As stated in the post, you can view currently cached domain names by entering
ipconfig /displaydns
Strangely, this doesn't seem to list some of the sites I've visited this morning within firefox
At which point the entry would still be cached by the OS, and since firefox apparently uses the OS API to resolve hostnames (if it didn't, I wouldn't have found the entry in my cache after opening the site, right?), it would still return the cached result (as long as it's not expired).
Besides, the question was how to prevent VAC from uploading your (hashed) DNS cache, and clearing Firefox cache doesn't flush those entries from your OS DNS cache.
First off, in that case I'm not sure what your experiment was supposed to demonstrate.
More importantly, Windows will cache DNS records for no longer than the TTL. Firefox will keep entries in its cache for hours if not days. That's how sites you have been visiting will not show up in the OS cache.
Proxy servers and/or added dodgy addresses into your [i]hosts[/i] file would circumvent it.
I'm not convinced I like other peoples suggestions of constantly flushing your DNS because you're having to hope that VAC doesn't recheck your cache before your automated job flushes it.