IPSec VPN Split Tunnel and Local Lan Access

No, not really. The access is controlled from FortiClient - you either use split-DNS, or route all the traffic through the VPN connection. You can add in split DNS the local subnet to match the network of your client (that you want to disable). This way the traffic will flow through the VPN, but reach nowhere (hopefully the subnet in your fortigate does not match the local subnets of your client, and that your client doesn't simply reconfigure the local network with a new IP range).