How to prevent update from conflicting repo?

How to prevent update from conflicting repo?

On EC2 / Amazon Linux AMI (Centos based), I created
/etc/yum.repos.d/nginx.repo:
[nginx]
name=nginx repo
baseurl=http://nginx.org/packages/centos/6/$basearch/
gpgcheck=0
enabled=1
Then I successfully installed nginx 1.4.2:
sudo yum --disablerepo="*" --enablerepo="nginx" install nginx
Now, when I run sudo yum update, I get:
---> Package nginx.x86_64 0:1.4.2-1.el6.ngx will be updated
---> Package nginx.x86_64 1:1.2.9-1.11.amzn1 will be an update
NO!
How do I permanently block the update? (which is actually a downgrade)