Can't change user's details with `usermod`, it claims they are still logged in

I can't change a user's home directory using usermod;

[root@server ~]# usermod -d /some/opther/path userfoo
usermod: user userfoo is currently logged in
[root@server ~]$ who
myuser  pts/0        2013-06-17 11:29 (1.2.3.4)

As you can see, I am the only person logged in, userfood isn't logged in anywhere.

[root@waccoe-web ~]$ ps aux | grep userfoo
root  16724  0.0  0.0 103236   884 pts/0    S+   11:36   0:00 grep userfoo

userfoo also has no processes running.

userfoo exists purely for FTP access to the server. I have stopped, started, and restarted the FTP server (ProFTPD) to ensure there are no hanging FTP connections. This server still thinks userfoo is logged in.

How can I either convince the usermod command userfoo isn't logged in, or remove any trace of that account being in use? I suppose I could edit /etc/passwd but that seems like working around the problem to me. Or is that infact going ot be OK? I would have thought that if the system believes the user is still logged in, this isn't going to work until all their sessions have been terminated.

[root@server ~]# cat /etc/issue
CentOS release 6.3 (Final)