I strongly suggest you to not only block him, but to also report him over here:
http://www.youtube.com/safety_help.
The more people who do that, the bigger the chance he will be taken down. By just blocking him, he will be able to continue his actions forever.
Also, be sure to not let emotions take over when reporting him. I think YouTube will delete all emotional and immature looking claims right away. Just to be sure (like.. don't call him a pedo in the report, just tell them what he said and that he harassed you).