skizzerz

It is impossible to completely prevent this, because the customer can always control what code is running on their machine. Below are some common ways to help protect client applications however. None of these are 100% effective (because it is impossible to be 100% effective), but are more to "keep honest people honest" -- that is, someone who wants to break your licensing will figure out a way to do so. People who are curious but see the roadblocks will be more likely to give up, however.
Encoding/obfuscation of source code Use HTTPS for your API calls and ensure that the server's certificate is strictly validated There are a plethora of other methods a quick search away, but the two above shouldn't cause much extra friction in your customers who are not trying to bypass your licensing. You don't want to make your anti-piracy measures so onerous that it breaks the application for paying customers.