There is not much you can do, if whitelisting the approved netblocks isn't feasible with your business. By that I mean permitting only the people you want to FTP on your server through the fw/router. Blacklisting becomes unmanageable, because you will continually be adding addresses, as I'm sure you've already seen. I would start with what you've already done and geo-block a few countries like the frequent offenders, but that also depends on your business requirements. Then make sure that server has a scheduled patching procedure, is monitored daily and probably chroot ftp if its unix. A more ideal solution would be key based SCP, but that all depends on if your business will accept it.

FTP Port-
It depends on the role of the FTP servers if you change it to a non standard port, your clients/customers may have a hard time connecting to it. Many corporate firewalls expect FTP to be on port 21 ( to start with ) and your clients/customers may have to work with their network administrators to connect to a non-standard port. So if there is a large number of outside clients/customers that use the FTP servers, changing the port may be more work than it is worth.

Thank you all for your help. I have a few ideas now.