The thing I'd do is deal with the condo association to replace the valve with one that works properly. There is no excuse to have valves that do not shut off completely. The only right way to fix this is to change the valve with a good one. I'm not sure I follow exactly what you have in mind, but it would be a bandaid repair at best. This is not a rocket science job, and INMHO, the association should fix it, if not you could easily do it.