The curse like effects of the stones are a proximal effects, much like radiation. The stones presence can affect more than one person, if the stones are large enough. The effects would be a continual test against the appropriate attribute, most likely self control or willpower (Methustine would be physical tests rather than mental) The curse like effects could be removed by simply removing the presence of the stone and time. Alternately, most magics that remove or negate curses could temporarily negate the effects. This would be temporary in nature because if the stones are removed, the effects would return when the spell expired.