You can get sand in warzone by using torches or slabs and letting sand fall on it.
This sand can be used for trapping players, making invincible defense, etc. which classifies as bug/exploit abuse, but since this bug is known for a while now, fix it?
There are many easy fixes:
- Prevent players from picking up sand/gravel items
- Prevent falling blocks entities from dropping items
- Make all blocks drop wood planks, including slabs and torches (or make them drop nothing)