Restrict IPSec VPN by IP

Hi Everyone,

I have an IPSec VPN configured between two locations. The VPN is working great.

I've noticed, however, under Log & Report > Events > VPN Events that there are a number of "IPsec phase 1 error" messages, all of which are from unfamiliar IPs. It's obvious that there some bad actors are trying to configure an IPSec VPN against our network. 

How can I configure the firewall to block any IPSec VPN attempts, except from known IPs in a whitelist we manage on the Fortinet appliance?