Maybe you'll just need to implement a number of days per round that posts need to be put in or that person's character is just skipped. In other words, a round lasts let's say three days. The players need to send in their posts during that timeframe or their character doesn't appear during that round. If a player misses too many rounds, then unless they have valid reasons, then that player has probably lost interest and might just need to be taken off. It works well on RWBY AU: Heroes of a Different Story.