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This is a clear sign the engine needs reworking. It can't handle too many players. If some players are removed it runs fine. Now would be the time for suggestions on how to separate the engine from the players.
As I said before, I think there is only one secure way - providing server-side API for the game.
The secure way would be to run the game in a separate process and make the players communicate with the game via IPC (such as over a socket). Not sure on the best way to do the IPC... perhaps some event processing of the pubsub variety?
EventMachine (perhaps using nats) comes to mind. Redis(alt) and 0MQ(alt) also have pubsub mechanisms.
This was fixed with #53