Do not "less emphasize" it unless it is a requirement!
Do not jeopardise your users' experience in the favor of discouraging an action!
These are two different buttons with two different functionalities that are EQUALLY important to the users.
Apparently users should be able to Register and to Unregister. Similarly I am able to buy from Amazon and I am able to make a return or cancel an order. Although Amazon would prefer less returns/cancellation happening, they do/should not make the Return/Cancel buttons confusing and less accessible.
*Less emphasizing does not mean confusing the user and making the task hard to achieve.
*There is nothing wrong with having the "Unregister" button replacing the "Register" button.
- Show something like "Already registered" label (with the check-mark maybe) for users who are already registered and coming back to revisits the page.
- Display the "Unregister" button in blue just like the "Register" button and remove the check-mark that you added next to "Unregister".
Now, if deemphasizing the "Unregister" task is a Requirement:
See suggestions in the update sections below.
I just noticed Mike's answer (I think it was posted a couple minutes before mine). I echo his idea: "Depending on the business goals, if you need to deemphasize the act of unregistering, you can perhaps make a subtle link".
END OF UPDATE (1)
This update is to suggest a design improvement based on the OP update and other answers:
END OF UPDATE (2).