How to stop IP conflict of another device and router/gateway on business network

Hi All

First time post, long time viewer. I manage a Fortinet network for an office of about 100 people, with one FortiGate, and around 15 managed Forti-Switches. About once a year, someone brings in a device that the IP conflicts with our FortiGate, which causes devices on our network to point to the wrong device when trying to get out the internet. I feel like this has to be an easy answer, but the Fortinet support team cant seem to wrap their heads around what I'm asking for. While I realize, long term, I can start adding several subnets over VLANs and also, changing the IP of the FortiGate(router) to something that maybe isn't as popular when it comes to default addresses, however in the meantime, isn't there something I could set on each of the managed switches that would make sure all devices would route to the FortiGate, for instance perhaps, via the routing table over MAC address? I appreciate any input.

Thanks!