A few points: janitors (editors) don't delete posts. Someone considers them for reaping, monks above a certain level vote on the consideration, the node accumulates the necessary negative reputation, and only then does it get reaped, leaving behind an ugly scar where the node once appeared. This may happen quite fast, or it may not. But it takes the intervention of at least four people every time it happens. Often spam-posts occur in rapid succession, making a sort of burst of activity required to keep up.