The strata manager should be asked to drop a letter in asking simply “are your renovations compliant with our by-laws?” The next line should read: “Please be aware that we have the legal right to compel you to restore non-compliant changes to common property (floors and ceilings, for instance) at your expense.
See if that gets anyone's attention – and at least they can't say they weren't warned when the NTC arrives telling them to carpet their floor and replace the ceiling.