You can't. It's not a two way street. Linking is always done through the third party back to Steam, not Steam back to the 3rd party.

The problem is this, when you link to Steam from EA, you're adding information to your EA account, not your Steam account. You can't manage your EA account from Steam. Steam support cannot manage your EA account. EA is not going to let Steam or anyone else fiddle with their user accounts. You have to do it through your EA account. If you don't have access to your EA account you'll need to recover it through EA support. And if you can't, you're SOL, EA is not going to make changes to an EA account you don't have access to or don't control.

And to add, if you get a ban on EA then you will never be able to unlink it, most linked accounts are doing this now to prevent abuse and to make bans effective too.