You asked this 1 year ago so hopefully this isn't too late - you can tell Nemo to ignore individual folder zoom settings by going to Edit -> Preferences -> Behavior -> and check "Ignore per-folder view preferences" (6th option). This is on Nemo 6.4.5. However, this will mean Nemo won't remember if you zoom folders, and will always show them as default zoom. I don't know a way to actually reset the cached zoom levels for all folders to default.