On OSX the launcher installs UE4 to here:-
/Users/Shared/UnrealEngine/4.0/
You can delete that folder fine. Also you can delete if you like the folder next to that:-
/Users/Shared/UnrealEngine/Launcher/
There is no real easy way to tell launcher it is not allowed to download the engine. You could just hit cancel (small X) next to the progress bar each time (which is irritating I know), but that will slowly accrue patch data each time if you are not speedy.
One way you could fool it if you like is to let it install successfully, and then just delete the /4.0/ folder as above… Launcher won’t try to download again if it thinks it has installed it successfully. There will still be a “Launch” button that won’t work 
Hope that helps…
Leigh