Equipment: new Goodman GKS9 furnaces. Conventional split systems: Condenser, furnace, heat pumps.

Here's the issue:

I do not want tenants to have the option to run the system with the fan = "on"...I only want the fan to come on when there is a call for heat or cool.

Can anyone suggest a solution other than thermostat lockout?

I'm sure this can be handled with a 24V relay, but I could use some polishing on which one and how to wire it in.