How to disconnect iPhone from WIFI without 'forgetting' it?
Turn off auto-join.  I’ve searched for this several times and always thought it was a stupid design by apple but I finally worked it out. 
In your iPhone settings, turn WiFi off then back on again. If it connects to your home WiFi automatically, click on the i (information) symbol next to the connection. Turn off the ‘auto-join’ option. Then you can turn WiFi off then on again to drop the connection without forgetting it. 
As the original post I’m replying to is five years old now maybe I’m the only one dumb enough to still be having this problem. But maybe not.  :)