how can i kick someone from a party

There is no kick function.  But leaving party (whether it was yours or someone else's) is simple.  Press options and then "leave party" is one of your choices, it's very similar to leaving a match.  Then set your lobby in settings so that players cannot join your group without an invite.  You can also set it so that your friends may join your lobby without an invite, but strangers cannot.

This sort of thing has always been a pet peeve of mine in multiplayer games where randoms can join others' lobbies.  Why on earth would I want random people who I don't know to have the ability to just jump into my lobby??

I made a small error here.  In order to change your lobby permissions, you must go to your friends list page in game.  And there is not an option to allow friends only to join without an invite.  It's an either/or proposition.  Privacy choices are "invite" or "open."