Unfortunately, one of TeamViewer's best features is how easily it works without firewall configuration. Often you can use it in schools and corporate environments. It is likely at least a little irritating to successfully block.

I found this (older) link, which seems to provide an IP range to block, and of course, suggests blocking TeamViewer DNS entries. But I'm not sure how good a block you'll manage on a Windows PC as opposed to a network device of some flavor.

http://mediarealm.com.au/articles/2014/10/block-teamviewer-n...

The easiest way to restrict the damage your parents can do to themselves is to make a separate admin user, and make them not an admin. Of course, then you volunteer yourself to install stuff for them too.

An easy way would be to route teamviewer's site to 127.0.0.1 in the hosts file, but that isn't fool proof.