hmm, well I know what you mean, I have a Nexus so its not particular to Samsung but I am on GiffGaff.
It connects to free open wifi but doesn't actually work without logging in, so you go to get data and the phone has no data connection via 4g because its swapped to the free wifi spot which is not active till you log in....annoying.
It doesn't bother me too much switching it off, the nature of my travels dictate that its a rare occurrence, but when it does I find that because I switched wifi off, the phone doesn't connect to my wifi at home.....which I do want ......annoying.
Anyway, I just checked my phone and I dunno if this will work but I do get forget options for 02 and BT with flippin FON and all the other crap, but I navigate to the list differently.
So
In the drop down window > wifi icon > more settings, in the more settings window is the settings cog top right, it takes you to the configure wifi window.
top of that list is saved networks, in there is a list of every network you've ever come in to contact with, on my phone selecting any of the networks listed brings up a forget option.