The Swordmage power Aegis of Ensnarement allows you to mark targets and trigger a reaction when the marked target attacks others.
Paraphrasing pertinent power text:
If the marked target hits with an attack that doesn't include you, you may use an immediate reaction (after attack is resolved) to teleport the target to any space adjacent to you, after which it grants combat advantage to everyone until the end of your next turn. If no unoccupied adjacent space exists, you can't use this.
It seems like the intent of the power is to grab an enemy who is trying to run amok farther away from you, in which case the text makes perfect sense. But I'm wondering about the (fairly common!) case where the target is already adjacent to you and attacks someone else anyway.
Can you use the reaction to teleport an already-adjacent target to a separate adjacent space and grant combat advantage?
Can you use the reaction to teleport an already-adjacent target zero spaces (leaving him where he was) and grant combat advantage?
If there are no unoccupied adjacent spaces, but the target is already adjacent, could you still do the previous case (teleport zero spaces and grant combat advantage)?
In general, I'd say yes. Specific answers to your questions:
Yep, absolutely. The power doesn't say the target has to start non-adjacent to you, so you can do it.
Trickier. I would say yes. From a rules-as-written perspective, a teleport doesn't have a minimum distance. On the other hand, the rules do say "A teleportation power transports creatures or objects instantaneously from one location to another." You could make a case that the second location must be different.
However, it doesn't seem reasonable to me that one type of swordmage mark can be made less effective in the one corner case, so I'd still say yes. After all, assault and shielding swordmages wouldn't care. As far as the flavor goes, it seems to me that vanishing from the world for a second might well be disconcerting, even if you returned to the same location.
In a pinch, you could teleport the enemy straight up one square. In this case, it'd get a saving throw, and if it made the save it wouldn't be teleported and thus wouldn't grant combat advantage. I think that would be legal even under very strict rules interpretations.
This is a subcase of #2, and all the same things apply.