Edit: I misunderstood what you meant. Second try...
You unload it from RAM by stopping the program. There is an easy setting to stop the program from loading at startup - in the Settings menu there is a box checked by default for starting along with the computer. Then, whenever you decide to game on Steam, you just spend a few seconds loading Steam up. I'd rather load Steam when I want it, instead of having it waste my time when I load my computer, as you also seem to prefer.
Options are useful, don't forget to look at them. Lots of programs do this "load with Windows" crap by default, but for many you can stop it.
Other steps you can take are going to mini mode (or whatever it's called now...the first version of the new style recently released didn't have the option that I saw, but another update put it back in I think), and going Offline from the Friends menu to save a very small bit of CPU / internet bandwidth if you aren't using the chat function.