Hello Simon,
Thank you for reaching out. You could use FeedItemService to mutate the feed items via Google Ads API Beta. The MutateFeedItems method will help you perform the remove operation by specifying the resource name of the feed item which is expected to be removed in the below format. Let us know if you need any additional information.
customers/{customer_id}/feedItems/{feed_id}~{feed_item_id}
Regards,
Nikisha Patel, Google Ads API Team