I hate when your sitting at the table playing BlackJack and some idiot decides to tell you that you played your card wrong or that you should/shouldn't have taken a card. Thing is half of these players don't have a clue about when you should and shouldn't take a card in the first place.
Three ways to go on this one
1. Memorize a mathmatical blackjack model and recant it to them as soon as they pipe up. Everyone at the casino will leave you the hell alone. Not showering also works.
2. Out-superstition them, If they say,
"you shouldn't be splitting twos or threes son"
Then reply along the lines of,
"Always split

and

when Margerie is dealing and the the daffodils are in bloom but only if you had two bowls of Wheetos for breakfast"
3. Pretend to be really interestes in what they've got to say. then next time the situation crops up, don't do what they told you. They will shake their head and mutter. This is good if you don't like them, as if you lose, it will reaffirm for them that their way is "correct" and they will continue to piss money against a wall at twenty-onsies.
4. If they're preaching the correct strategy, tell them they're wrong and that the correct strategy is to not play Blackjack. You play for fun and can do anything you darn well like. Next time you have 11, against a dealers 6, instead of doubling down, just stick. The table'll go nuts, it's dead funny.