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.