Remind yourself that they don't get it. This one is pretty much a catch-all. The other points in the list are just back-up plans. If a playtester doesn't understand your game, it's probably because they are not as smart as you. Even if it is a valid point, it might also be fixed in the future, so don't bother listening.

Clarify to the playtester what they didn't understand. Okay, so they're dumb enough to come up with completely bizarre misinterpretations of what's obvious to any game designer, but if you just tell them what you were trying to do, and explain what you're thinking, that might help their feedback get closer to the "you're awesome" zone.

Show them how to play it correctly. If they STILL don't get it, just take the controls and demonstrate the right way to play. This is the only way to ensure that the game plays exactly how you want it to, and besides, it will be educational for the tester that watches. When they get the controller back, maybe they'll start to get it.

Are they younger/less experienced than you? Another great sign that feedback is useless is when someone is just a kid. They didn't play your favorite obscure RPG on the original PlayStation? Useless. If they haven't 100% completed Dark Souls, or watch too much Youtube, their brain is probably mush anyway. No need for their feedback.

Are they older/irrelevant? Look, let's just come out and say it. As a game developer, you're in tune with the target demographic, right? Anyone not exactly the same age as you just won't have the right vibe. Older people do things like reference Atari games, or complain about Youtube culture, and thereby show that they are completely out of touch with the modern reality. Their opinion on your game can be safely ignored.

Don't let them play your game. The best way to avoid unwanted, useless feedback is not to let anyone play your game! As soon as those grubby mitts touch the controllers/keypad/headset, your grand vision will be reduced to a bunch of bleeps and bloops, and these mortals will have the audacity to critique your beautiful creation! So when you make your game, stow it away in a hidden folder. I like to name my secret-games folder "TAXES" so that even my partner leaves it alone.

Don't make games at all! The thing about secrets is that they tend to get out. So unless you burn your harddrive after making the game, eventually someone might play it and ruin everything. So the only sure way to not get useless, annoying feedback is to not make any games. Problem solved. At least you still have your journal of ideas, perfect and untouched in your dreams.