On my 720, the way to get rid of the (Pre) networks is to go into the wifi setup, select the network, then click "Delete".
Due to the aforementioned "quirks", this can be tricky, because if you're not in range of the (Pre) network at the time, the "(Pre)" network names may appear and disappear, so they can be tricky to select. If you don't see a (Pre) that you think is probably there, try turning wifi off and then back on again. On my 720, that causes the "(Pre)" network names to appear for a second while the device is scanning the airwaves for other currently-available networks. Once the currently-available networks list populates, the out-of-rnage (Pre) networks are no longer listed (though it might toggle between those two lists a couple of times before settling out).
If you have several (Pre) networks defined, you may have to toggle wifi off/on several times to get the opportunity to delete them all (except for the one you use in your plane--your goal is to have ONLY that one network defined as "(Pre)").
Like I said..."quirky".... It's why I only have my Stratux network defined and don't mess with trying to connect to any other networks. Just not worth the hassle.