Under the settings in your profile, there is a nifty "Quit HOL" button that would allow you to deactivate and effectively quit your account.
However, this button should be pressed sparingly, if at all. It is difficult to "un-quit", and getting hold of an admin to re-activate your account will probably result in much head-bashings and muttered expletives. Therefore, to save the admins trouble, do this when you're ABSOLUTELY SURE you want to.
Becoming inactive is another option that's more preferable. Not earning points will turn your account into a passive account, and thus will not be part of the House Points tally. If you ever feel like getting back to becoming active in HOL again, merely earn a few points in a particular term and voila! You're back as an active HOL member.