How do I remove someone from sharing my ICloud

Change your Apple ID password to a new and unique password, and log into iCloud.com and check for and log out all but your devices and computers. See Use the web to see where you're signed in here: Check your Apple ID device list to see where you're signed in - Apple Support

Effectively, your Apple ID has been compromised. How much effort you will want to take securing your Apple ID here depends on how malicious you expect the other party might be. Here is what to do If you think your Apple ID has been compromised - Apple Support

Tell them to create their own Apple ID -- How to create a new Apple ID - Apple Support.