You can do so by setting session limit. Below are the sample and commands:

CLI Commands:
(config)#virtual site session reuse off <virtual_site_id>
(config)#switch <virtual_site_id>
(config)#session maxperuser 1

CLI command Example:
#show run virtual
virtual site host "vip5" "www.myvip5.com" 10.7.13.10 443 "exclusive"
virtual site session reuse on "vip5"

Set limit login using CLI
(config)#virtual site session reuse off "vip5"
(config)#switch “vip5”
(config)#session maxperuser 1

WebUI Configuration:
At Base system, in config mode
1. Click on virtual sites  -> at virtual sites tab -> uncheck Session Reuse
2. Click on Service Management -> Session Limits Tab -> click "Add" at site session limits -> select Virtual Site ID and enter Maximum number of concurrent active sessions -> click save