ThalysNet or other accesspoints destroy my browsing experience
Disable "Ask to join new networks", when selecting Wi-fi in the Network preference pane. You might also need to explicitely forget those networks if you joined them in the past, by clicking on "Advanced..." and removing any undesired network.

Once you are connected to these networks, there is no way in changing this behavior. As every request passes through their access point, they can modify it in every way they want (and return a redirect response instead for example).