You are right, there is not a 100% chance of it not happening if 100% of the nodes don't have the filtering but the percentage that blocks mined by a miner that do not conform with the nodes' rules get rejected and becomes an orphan block go up as the number of nodes validating that block based on those rules goes up. We don't need 100% of the nodes to have spam filtering, we just need that minimum critical mass where there is a strong chance that orphan blocks are created - which should start incentivizing miners to comply a bit more with the more restrictive parameters to avoid loosing all their effort in an orphan block.