UK citizen who moved to the USA for 1 year. Moved out of the USA in August 2020. It is adding a stimulus payment for Dec but I didn't live in the USA. How do I remove it?

You do not need to remove it. The IRS is issuing you the second stimulus check and this is based off of the mere fact that you lived in the US for any period of time during 2020. The second stimulus checks distributed was an "add" to the first one for the year 2020 (and the first was based solely on 2019 which is why you most likely did not receive that one). You are fine - it's money allocated to you by the regulations. If for some reason the IRS receives your return and decides further you do not qualify for it, then they will not add it to any refund that you are due. That is all.