Hey Ben,
You can hide content from certain users but you cannot ensure security on anything in your doc. If the doc is shared with a particular person even with read only privileges, consider that the whole doc is shared for them to copy and explore. The only way to make sure that only part of the doc is shared with someone is to sync that part to a separate doc with crossdoc and share that other doc.
Otherwise you can set up filters on your tables and use something like the DB Flows pattern to manage who can see what. I also talk about this more in the 1st part of my Best Practices Showcase series.
Oh, and welcome to the Community! 