Creating a UNIX account which only executes one command

You could used a forced command if the users can only connect through ssh. Essentially, whenever the user connects through ssh with a certain key and a certain username, you force him to execute a command (or a script or) you determined in the .ssh/authorized_keys. Commands issued by the users will be ignored.

For example:

# in .ssh/authorized_keys
command="cd /foo/bar && /path/to/scripts/my_script.sh arg1 arg2",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty ssh-rsa public_key

You could set the shell of that user to a script just running the command you want to allow: Whenever the user logs in, the command is run, then the user is logged out. And since there's no "full shell" you don't have to deal with the user trying funky stuff ;)