How do I delete messages on MacBook Air, but not on iPhone 11 Pro Max??

I cannot figure out how to delete iMessages on my MacBook Air 2018 to free up storage? How do I remove the 30GB+ file from my Mac without removing them from my iPhone 11? I have both devices set to save in iCloud? I haven't been able to find any clear answer to this problem. I'm running MacOS 10.15.5.