Someone else linked their EA account to my Steam now I can't play games.

Since this is an account-related issue, I suggest that you contact them via chat: 

How to chat with EA Help

1. Log in to your EA Account.
2. Select the topic that best describes your issue. If an advisor can help, a contact option will be available under your help topic.
3. Select Chat with us or Start chat.
4. Fill in the contact form with your:
   • first name
   • last name
   • e-mail
   • platform
   • subject (a short description of your issue)
   • attachments (e.g. screenshots, documents, links).
5. Select Send.
6. A chat window will open when you’re transferred to an advisor.

After you contact us, we’ll create a case for you. You can view it and keep track of updates in My Cases. You’ll be notified of changes to your case via email.