turn off icloud when you no longer have the device

Do I gather you have moved to a new iPhone and are using iCloud Storage on that? In that case it's the same storage (the storage is online, not on the phone) so you don't want to turn it off.

If however you really don't want the storage - for example if you've moved to Android - then follow the instructions here:

Downgrade your iCloud storage - Apple Support

This will downgrade you to the free 5GB level but of course does affect any device logged into that iCloud account.

The page I linked to tells you how to downgrade iCloud storage to the free 5GB level using a Mac or PC. The storage is not specific to your old iPhone - it is accessible to any compatible device signed into that account (not Android, of course). It's not an 'iPhone account'.