If the spaces are designated as car parking spaces on the plan then it’s reasonable to assume that that is all they are permitted to be. If they ar designated as car parking and storage, then you have a different problem.
Then you need a by-law specifying what can and can’t be kept in car spaces (regardless of whether they are lots or on common property). The issues are fire safety and the look of the garage – nothing depresses unit prices quick like a garage space that looks like a tip.
My building has very strict rules and storage is only allowed in a box thing that’s called, conveniently, the “Box Thing”. Goods can be stored out of sight but cars can park with their bonnet under the box.
You may already have a by-law covering the guy who has stacked his space but I would think your fire safety inspectors would have something to say about it.