My macbook air startup disk is full and I want to delete some old emails/photos etc but these keep coming back onto the computer fro micloud. I have things saved on another desk top so I just want to clear things on my Macbook.

My macbook air startup disk is full and I want to delete some old emails/photos etc but these keep coming back onto the computer (presumably from iCloud) even after I delete them from the trash. I have things saved on another desk top so I just want to clear things on my Macbook so I can use it again. I have quite a few devices which use the same Apple account. I would like most of the data to be stored on my iMac and in the Cloud, but not on every device. Thanks!