The key connection isn't the folder - it's the collection. So look for those photos in the synced collection and remove them from there. To find it, right click a photo and there's a list of the collections in which it is listed.
Also, in the Catalog panel, see All Synced Photos - you can remove them from LrMobile there too.
PS copies of the photos will remain in iOS's Photos app. As LrM can't remove them from there, you'll have to do it manually.
All your images are saved on Cloud and are managed directly from the catalog on your Desktop.If you remove an image from the synced collection itself on the desktop that should remove them Lr mobile as well on all device.It will show a pop as shown below.
A quicker way is to delete the Lr mobile data.In Preferences > Lightroom Mobile, Choose "Delete all Data" button.Note this will delete All synced data to/from mobile.
Note this will delete All synced data to/from mobile and that cannot be recovered.