OK, regardless of what the owners say, they are in breach your by-laws and that has to be dealt with, by issuing a Notice To Comply, if necessary. If they can prove the Real Estate agent sold them the property on the basis of a false promise, then they should take that up with the RE agent. But that doesn’t alter their obligation to abide by by-laws
Meanwhile, I suggest a strongly worded letter from your strata manager telling the RE agent that if they continue to mislead potential purchasers about the illegal use of common property, you will report them to Fair Trading and the Real Estate Institute with a view to having their license revoked..