I'm helping a half-dozen players on a private Battle.net gateway through a small noob tournament I set up (to get some activity on that private gateway). Unfortuantely, there's one rather annoying player who doesn't take any advice and just masses battlecruisers. I've beaten him several times doing something different each time (rushing, defiler/hydra, mutalisk/devourer, arbiter/scout, goliath/ghost/EMP), but he still even says that he doesn't need advice and he's playing perfectly fine.

Any way I can get it through his annoyingly thick skull?

No. Tell him to fuck off.

Or you could do it by using the same strategy as him, more or less.

Let him realize his own error, keep using the same tactic to beat him until he thinks for himself. Don't mention his errors. If he continues to mass Bc's, he obviously doesn't want to learn, and I would just kick him out.

Alright, thanks.

Meanwhile, through my replays, the other players in the tournament know how to beat that strategy anyway. I've rushed him as Terran, Zerg and Protoss, I've gone Hydra/Defiler, Muta/Devourer, Scout/Arbiter (which was hilarious), Dragoon/High Templar/Dark Archon, Goliath/Ghost and mass Wraith against him. He either needs to follow my advice and do new stuff, or keep losing

Tell him that if he wants to win then listen to you, he obviously doesn't care for anyone's help just let him play like that and when he complains about losing then tell him i can help you but don't build bcs >:O

Just asking if he's still not willing to listen and still is using the same Battlecruizar mass tactic.
If he still is, I would use this opportunity to get creative with your tactics. Something like blinding detectors then cloak wraith, massing storm, Mind controlling all Battlecruisers, or even just trying a build that you know will work since he rushes BC and doesn't have any other unit out. Actually, i find it pretty rare for someone to be so thickheaded and continue to use the same tactic even though he does realize that it's worthless.

Massed battlecruisers supported by a decent mech army is actually fairly hard to stop, so the main problem here is that you guys are letting him get to massed battlecruisers. Teach him to fend off rushes first by beating him with the same tactic every time until he does something different.

Alternatively, kill all of his battlecruisers with a bunch of microed goliaths.