Base stealing runner...how to get him out?

Wrong. If a baserunner breaks for second, the pitcher can throw either to first or second unless he's started his pitching motion toward home. So it's not a balk. If a baserunner is on second and breaks for third, the pitcher has the same options. He can throw to either base unless he's started his pitching motion toward the plate. Again, it's not a balk. That's why you seldom see a baserunner make that kind of mistake. It's baserunning suicide.

Two approaches:

1) Pitcher throws to first, with or without disengaging the rubber, and first baseman throws down to 2B.

2) Pitcher disengages the rubber, then throws to 2B. It's important for the pitcher to disengage because, if the runner is bluffing, and the pitcher throws without disengaging, it ends up being a balk for throwing to an unoccupied base, per rule 8.05(d).

Since the batter hasn't batted the ball, there are no force plays to be made, and the runner must be tagged.

Once the pitcher steps off the rubber, he becomes an infielder and can throw to any base.

Only if the pitcher stays on the rubber, he cannot throw to an unoccupied base.

If the pitcher hasn't started towards the plate, he can step off and throw to second.

ya the first answer explained it exactly right but to answer your question if he must be tagged, yes he does. cant just step on the base because its not a force situation. if he is on first and a groundball is hit, you can get the guy out going to second by stepping on the bag because the hitter is running to first, forcing the other guy to advance, hence "force out". If he is just stealing second, nobody is forcing him to advance so he must be tagged.