To remove a Trusted Network, head into your TunnelBear settings. Under Trusted Networks, you'll see any networks that you have told your Bear to trust. If you want to remove one, swipe left on the network name. This will expose a Remove button. Click that and the network will be removed from your trusted network list.
iOS: Trusted Networks
When the network is removed, your Bear will no longer stand down when you're connected to that network. This will allow you to connect to TunnelBear on that network as you would any other.
To learn more about the Trusted Networks feature, see this article.