Did you configure the mount of your NAS in our Filesystem/Mount menu? If so, you should be able to remove any mountpoints in there with a button.
The behaviour you described should not occur with a usual mount. Please check what files and folders are in your mountpoint and whether there might be some special rights or files in there.
Please also try mounting a share that only has a few simple files to rule out the possibility of the share’s contents as the source of error.