You can limit the number of sessions on the domain here.

Max Session
Limit the number of sessions logged on this domain simultaneously.
Max Session Per IP
Limit the number of sessions from the same IP.
Anti-hammer Enable
Activate, deactivate anti−hammering.
Send hammering message to ftp client
If a client tries to hammer this server, the server will send a message to him/her.
Ban IP xx seconds if xx failed login tries in xx seconds
The counter will monitor the login tries and count them, if the number of failed login tries during the period is exceeded, the user IP will be banned for a period of time as defined.