There's a lot of great debate over what's considered kosher.
I've seen spawn-trapping broken down into two categories. I actually perform one of these.
The one I perform:
To me spawn-trapping is best seen in Dom. Taking A/B and keep the enemy at C or any other variant of that mix. (BC/A, AC/B)
The one people don't like:
Abusing the game spawn mechanics with various tools to force users to spawn in fixed locations. You'll find this normally in Demo or Sabotage. Essentially, the aggressor is standing in a location where the victims will spawn in their sites or directly in front of them prime for the killing. If anything, this is most certainly against the rules.