Can you get banned using third party apps to find out answers?

I don't think you can get banned unless you are using Kahn academy through your school, in which case your teachers could probably ban you

Hi Laurão,

Using Khan Academy for any form of academy dishonesty or attempting to modify the website's functionality are both prohibited conduct according to our Terms of Service. So yes, your account could be restricted or deleted for doing this.

If you know someone doing this in-person, then I suggest reporting the behavior to a teacher so they can take the appropriate actions.

I hope this helps!