I bet you could use either:
[cmd=]tar cf tarfile.tar --one-file-system /
tar cf tarfile.tar -W one-file-system /
[/cmd]
BTW, be careful to not put the tarfile.tar on the same mountpoint as the one you are tarring up. At least with older implementations, it would try add the tarball to the tarball, and well, fun things happen in that case.