Well, since all Edu organisations should (since long) all have switched to Block Everything, most shouldn't have to do anything to stop users from using Login with Google to unsanctioned services.
But those who already have allowed OpenAI and now has changed their mind, you can remove it from your list of Trusted apps. Granted, blocking it, as a first step, is often faster.
The client_id for OpenAI is 799222349882-ne3i0s9jdm5s0p7ll2d7tlsi1vc1halt.apps.googleusercontent.com
Today, the name is actually OpenAI, but in the beginning they called it Universe. Completely useless description.
Removing an app from the Trusted list.
There is no easy click to remove button. Believe me, I have asked for it! 🙂
Instead what you can do is Download list of currently configured apps, adjust the list, and then Bulk update list.

What you do is change the value in column Status to UNCONFIGURED.
When you upload the list again, it'll remove the app from the list of Configured apps, logically. 🙂
You should probably not keep rows of the other apps in the list to upload, else it'll try to change those too.
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