How to block the facbook app but not facebook messaging?

So I have done the parental controls and blocked Facebook and it says messages are not blocked. Well the Facebook app is still able to be searched in, commented in, and stuff. The fb messenger itself is now not working but actual FB is. I've un-installed the facbook app but not messenger and they keep downloading the fb app. So I want it to know if there are certain domains that need blocked in the denial list on dns for the facbook app and are there certain domains for messenger that need to be listed in the allowed list. They are looking at inappropriate things on FB and commenting on them as well. I've even tried family link with no success.  I'm one confused mom, if anyone could help would be greatly appreciated.