Well, you should consider what other traffic is needed to go through the pix, prolly not just ftp. But anyways the command would be something like :
! Permit ftp out !
access-list inside-out permit tcp host 192.168.x.x host 16.x.x.x eq ftp
! Bind access-list to incoming traffic on the inside intf !
access-group inside-out in interface inside
Now nothing other than ftp from that address will be possible.