delirial – I disagree with some of the other posts, in that your Plan’s existing By-Law states that nothing may be placed or hung on balconies that’s inconsistent etc etc, and the resolution of the Owners Corporation merely prescribes what is consistent with regard (only) to the design and appearance of privacy screens.
Given the plethora of “things” that could conceivably be “placed or hung”, that’s an entirely logical approach in my opinion, and what you need to do in order to resolve the issue of others placing other types / designs of privacy screens on their balconies is to remind your Executive Committee (E/C) of their obligations, and request the Secretary and/or Strata Manager to advise those Owners in writing of what the O/C has resolved is consistent with the aesthetics and appearance of the building, and instruct them to immediately remove whatever is not.
If as you suspect your E/C is unprepared to do that, then Jimmy’s post #3 sets out what then needs to be done.