We had an analogous situation. A unit owner had put up a fence enclosing part of the common property into an extended unit courtyard. The fence also did not comply with any of our several options for fences that may be approved. The owner had stonewalled ECs for several years. Once I was on the EC I proposed that we get serious and deal with it via a breach of rules (ACT word for by-laws) notice. By chance this coincided with the owner deciding to sell. We wrote to the owner, explained that it would be a good idea to comply before selling because the breach notice would be on the EC records available to any prospective buyer to see and it might be a problem for the buyer looking to get a loan. This was ignored.
I talked to the selling real estate agent who was really helpful. I gave them a copy of the breach notice and explained what would be required to correct the breach and what sorts of fence would be acceptable in what locations if a new owner wanted one. The agent informed each serious prospective buyer of the situation and the new owner did promptly remove the fence, preferred to not have a new fence and instead landscaped the area very nicely. A nice result.