I just paired my set of Bluetooth headphones speakers with this TV in order to answer this question. I have my Bluetooth headphones pair with two of my other Samsung TV. However, in order to provide an honest answer ... I wanted to try the headphones with this exact TV .
The answer is once your headphones are paired and connected YES it will cancel the internal speakers.... You will only be able to hear the sound through the headphones while the headphones are connected . A very nice feature to have . I use it all the time while watching TV in bed as not to disturb my wife while she is sleeping or reading.
I hope this helps !