It is only about 2 users who spam that board constantly by promoting two sites, and few others just use this board for sig spaming. So forum only need to handle with 2 users, and intruduce new rule on that board, problem solved. If they continue to work with new accounts, they will be reported and banned in a matter of a few days.
I think that could be a better solution then to close that board, especially if it is just about few users that in some way abuse forum because of its private interests.