The simplest and safest way is to switch of iCloud photos (in photos preferences, iCloud tab)
And then delete the whole library from the pictures folder. If you are SURE that all your pictures are on your phone.
But then you have to think about how your photos will be backed up. iCloud is not a backup - if you accidentally delete photos off your phone, they will be deleted in iCloud also, and they will be gone.
You may be better off moving your mac photo library to an external drive, keeping it synced with iCloud, and then backing it up using time machine.