Apart from the "huntee" having a three-hours' head start and having a knife, there aren't any. It is an unfair advantage from the start and becomes even moreso when Zaroff gets help from his giant butler Ivan and then from his dogs.
Rainsford outwits Zaroff by jumping off a cliff and swimming back to the castle before the colonel can returns with his dogs. Zaroff presumes his prey is "lost" and will die at sea; he is shocked to find Rainsford waiting for him in his own bedroom!
Rainsford knows the "game" won't be over until he has killed Zaroff simply because there are no "rules"; the deadline of three days before his giving up won't be respected, either.
The reader understands in the closing sentence that Rainsford has not only killed Zaroff out of self defense but that he doesn't suffer from any guilt pangs. He has saved his own life and delivered others from a depraved maniac. That's why he sleeps so well that night.
The rules to Zaroff's game are deceptively simple. Zaroff first suggests to his victim that they go hunting. The unlucky candidate can choose not to, but if he will not take part in the game, he is turned over to Ivan, who "has his own ideas of sport". The result is that there really is no other viable option, and the victim "invariably...choose(s) the hunt".
The chosen subject is then given a supply of food and "an excellent hunting knife". He is given three hours' head start, after which Zaroff will begin his pursuit, armed "only with a pistol of the smallest caliber and range". If the victim manages to elude the hunter "for three whole days", he wins the game, and he is given his freedom, taken by sloop back to the mainland and released near a town. If he does not elude Zaroff, he "loses", and is killed.