Chad,
Your Spa spillover should only work when commanded.. It should not be on all the time. Your Return valve is either not positioned correctly or you have a "Make up" valve that you should close...
Without automation, unless you turned a valve no fresh water would flow to the Spa.. To fix this, either a make up valve was added that would force some water to the Spa even in the Pool mode... When the pool builder was too cheap to install a make up valve, he would just misadjust the return valve, so that some water would always go to the Spa...
Moving in the this century.. pools with Automation are often installed by the old time pool builders... forcing the Spa into continuous spillover..
With Automation in the Pool mode, or the Spa mode, you should have zero spill over. If you want the Spa to spillover, you select the Spillover mode... This way you only have spillover when you want. And to keep the Spa water fresh, you can just schedule the Spillover mode to turn on a couple of times a day for half an hour or so.
Thanks,
Jim R.