Listen for a glottal stop at the end of "can't". It'll basically sound like the <n> got cut off, rather than completed as a sound.
In most American dialects, "can't" is pronounced with the stop when in it's followed by more information: "You can't do that."
It's pronounced with an enunciated /t/ at the end of a sentence: "You can't."
Both of these are assuming normal-speed speech. Faster speech tends to lose the /t/ and you get the glottal stop again, while slower speech tends to emphasize the /t/ .
As for negation: We often use double-negatives to emphasize something. You can say: "You have to go" and be implying an obligation, or you could say "You can't NOT go" (yes, emphasis on "not") and be implying that there is no "not going", no getting out of going. You literally have to, where the first one is just saying that you're expected to.
Double negation is very handy for speeches and essays. And a great way to mess with people who don't expect grammatical twists. An (slightly inappropriate) example: "Aren't you not a virgin?" A trap! Think about how you were asked before you answer that, lest you tell an untruth.