Joe, If you ever want to update your operating system, you must have an AppleID account to do that. If you don't want an AppleID, go to Apple.com/Support and follow down the prompts until you get to schedule a phone call. Apple will call you back, usually within minutes, and tell you what can be done. But generally, you'll need an AppleID to keep your machine updated with security updates as they arrive. And with that AppleID comes access to certain iCloud features, like sync between Apple devices. Storage in iCloud is one of those services, but you don't have to use it.
Before we talk storage, do you have more than one Apple device? I ask because if you have more than one, they synchronize through the iCloud services. You can stop storing stuff at iCloud, but you still need that AppleID to get updates to software and access to the App Store. And to stop storing at iCloud, you would have to turn off the feature on every device. For example, if you have an iPhone and take pictures, you would have to turn off synchronization of Photos, and PhotoStream, to stop the images from passing through iCloud to get to the iMac Photos or iPhoto. Ditto for word processors, mail, messages, texts, iTunes music, video, etc. It can be done, if you really don't want to do it. Lots of work, and it sort of diminishes the advantages of being in the Apple ecosphere, but it can be done.
The Apple iCloud is very secure. Remember when Apple couldn't help the FBI with the terrorists on the West Coast? Apple takes security seriously. Now, if you don't want to know about iCloud being full, you CAN stop storing things there. On the iMac, open System Preferences, iCloud and see the services that are using iCloud. Uncheck any and all that you don't want and it will stop. And when you create a new document or file on the iMac, when it asks to save, make sure you select a location on the internal drive and not an iCloud location. Do that on all of your Apple devices that share your AppleID. Otherwise, any device still "sharing" through iCloud will continue to push things there and you will continue to get the notes about being full. You can have an AppleID and not store anything at iCloud, as I said.
Frankly, in my opinion, not using iCloud is a bit like putting square wheels on your car so it won't roll away when you don't set the handbrake. You lose a lot of functionality for very little increase in security. But you CAN do it.
