This is a fairly common question.
For some contests, the commissioner simply wants to limit the players to reserving only one box per player.
For more complicated contests, commissioners ask to be able to control the number of squares available to each player on a case by case basis. For instance, after a player pays for 4 squares, the commissioner wants to "release" exactly 4 squares to the player.
Unfortunately, the current architecture at SBPS does not allow for a limitation on the number of boxes a player can reserve. This is simply because the only roadblock between a new visitor and reserving a square in the contest is whether or not that person knows the Contest Password.
Even if we were able to limit the user from reserving two or more boxes in a particular contest, there would essentially be nothing stopping that person from creating a second account with a different email address, continuing to reserve one additional box per email address.
For these reasons, no such limitation exists in the current architecture of SBPS.
We suggest a more open approach, allowing players to pay for and reserve as many squares as they like. You can always copy your Contest into a brand new Contest with the same settings, to add another 100 squares for players to snatch up, while increasing the size of your prizes and/or fundraiser dramatically.
We do have future plans in the works to offer a more robust solution to commissioners, giving individual control over who can reserve additional boxes in the contest, and hope to release that sometime in the future.