According to the compiled Q&A for spirit companions:
Q; How far away can my SC go and still be sustained?
A: It needs to be within 20 squares of you at the end of your turn. There is a lot of argument about this, but for my answer I am relying on this FAQ from the Player's Handbook Races: Dragonborn, which states that "Q The Dragon Storm power has a zone that starts in a close burst 2, but can be moved. How far can I move it? A: You can move the zone 3 squares, and it ends if you end your turn more than 2 squares away from it " So as poorly as this answer is written, we now know that close burst powers do have a range requirment for sustaining. The argument for why this is controversial is in the sblocks below and I think the FAQ and the info from Dragon 387 is good enough for determining RAI.
Q: Do I need line of sight or line of effect when using spirit powers?
A: Normally with conjurations you need line of sight to use a power, but with SCs it is different and you use your SC to determine line of sight. You determine also line of effect from your SC. See page 220 of PHB2
On the debate on close burst powers:
So that's why people disagree about this and why we need an official FAQ: page 56 makes it look like close burst 20 is a range, page 270 makes it look like close powers do not have a "range". Ask your DM, but in my experience this does not come up too often in real game play. No matter how far away you think the SC can go, you need to have line of effect to it at the end of your turn or it goes away (see PHB2 page 220).
So, Loss of LoE and LoS to the companion will pop it.
Under the most recent rules updates echoed in DDI, we get
Effect: You conjure your spirit companion in an unoccupied square in the burst. The spirit lasts until you fall unconscious or until you dismiss it as a minor action. The spirit occupies 1 square. Enemies cannot move through its space, but allies can. When you take a move action, you can also move the spirit a number of squares equal to your speed.
For other ways, it would have to be an anti-conjuration ability built into the enemy like the Firequench Wizard
Area burst 2 within 10; +9 vs Reflex; 1d6+4 damage, and the target is pushed 2 squares and knocked prone. The area is difficult terrain until the end of the firequench wizard’s next turn. Normal fire is extinguished within the area. Magical conjurations and zones of fire are extinguished if the firequench mage hits with the following attack: +10 vs. the Will of the creator of the conjuration or zone.