unmounting a NFS-exported file system

[Edgar Fuß]

I would like to manually fsck a filesystem which is currently NFS exported.
I could stop NFS, but that would affect several other file systems, too.
Is there a sane and safe way to handle this, i.e. resulting in being able to 
fsck while NFS clients just hang with "NFS server not responding" for that one 
file system, but not others?