The condo association in this type of situation is legally obligated to adhere to their own internal by-laws as well as the policy and procedures that are in place for this particular condo. The rules have to be in compliance with state law as well. As a result, most condo association by-laws look strikingly similar.
That being the case, it is possible, likely after a board vote, that the rules could be change as to availability of the pool. As you have stated, in most cases though, this take a vote from the majority of the condo owners - not just a fiat ruling by the condo association board.
So, what is the remedy for you and those others that are aggrieved by this decision? First, you need to review the bylaws that govern your particular condo and condo association to make certain that they are violating the bylaws. If they are, then you can file for an injunction against this ruling on a court in your county of residence to would effectively void the ruling of the association. The association board could then try to go about this the right way and get a three quarters vote from the condo owners if they want to re-implement this rule.
Before going to court seeking the injunction, you and those others aggrieved should send a certified letter with a return receipt to the board in which the explain the issue, point out their violation of the bylaws and request that this be changed immediately. They will likely not comply, but you can use this as evidence of legal notice and your attempt to resolve the issue without the courts (which must be shown to get an injunction).
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Best wishes going forward!