When connected to a Wi-Fi hot spot, especially if the hot spot uses multiple access points and there are multiple networks in the vicinity, you may encounter a problem where your Mac keeps losing its network connection and then attempts to establish a connection with a network for which you do not have access.
Additionally, even after you have selected a working connection, if you put the system in sleep mode, restart it, or if you just temporarily lose your connection for some reason, the system will again attempt to connect to that network. Having to continually select your Wi-Fi connection from the Airport menu can be a pain.
Usually such problems happen if you have inadvertently connected to a closed captive portal network in the vicinity when selecting a Wi-Fi connection. In this case, the system is able to establish a preliminary connection with the captive portal, and therefore saves the connection attempt in its preferred wireless network list.
Once in the list, if this network is given priority above others, it will be the first one that the system connects to, resulting in you continuously seeing the active portal login prompt.
To remedy this situation, you simply have to rearrange your preferred networks, and remove any from the list that you cannot use. To do this, go to the Network system preferences and select your Wi-Fi connection. Then click the Advanced button, where you can see a list of the preferred networks in the Wi-Fi tab. Drag the networks in the list to prioritize them, placing the preferred ones at the top. Then locate and remove any networks you do not (or can not) use.
In addition to setting your preferred networks, you can lock the system to one by checking the box to require authentication when changing networks. This will issue an administrator prompt if you lose your connection and the system attempts to connect to another network in the vicinity.
Once finished, the system should reconnect to the networks using these settings, and you shouldn't have to manually switch them as much.