how to disable specific ip address in ISDB

Just create an loopback interface with a ip address. (Can be a /32) Change the sslvpn interface to the loopback interface you just created. Then go to vips and create a VIP with your external IP Address and forward it to your loopback ip address.

If you want to have a search bar in the GUI, I would suggest to reach your local Fortinet representative and ask for a New Feature Request.

For now you can try the CLI approach 'config firewall internet-service-extension' as shown in this article.

Another approach is to add a firewall rule to the specific IP just before the ISDB related rule, in order to allow or deny the traffic to that IP address before it is matched by the ISDB related rule.