Islanding is a very unfair yet common spleef tactic, where one player separates himself from the rest of the spleef arena in a way that puts him on an "island" which cannot be reached. This can be fixed if both players type /reset to reset the spleef arena, but islanders often refuse.
There are four main ways to island:
Corner islanding: crouching in a corner and digging up all the blocks that you can reach; easily the most common form of islanding.
Section islanding: cutting a gap in the arena so wide that it can't be jumped across, creating two separate sections.
Circular islanding: standing somewhere in the arena and digging a thick circle around yourself, leaving an island surrounded by snow. The island may be only a single block.
Forced islanding: an island which is formed as a result of all the possible paths to it being cut off by the player not on the island. This is often accidental, so just type /reset to fix it.