DNS filtering question, blocking VPN traffic: Advice appreciated.

Question: I am new to the network admin field. Like brand new. I just graduated in May and am volunteering to manage my Church network to help them out and get some good experience.

I have implemented DNS content filtering through cleanbrowsing.org to limit access to objectionable content on our network. During the setup, I have linked our public IP address to my cleanbrowsing account.

My two questions are:

What would be the best way to block users on the network from using their own VPN software...firewall?

Also, how can I deter the use of other DNS servers other than inputting the appropriate cleanbrowsing DNS servers into the router?

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Is there anything else that I'm not thinking of in this implementation?

Also, I'm up for constructive criticism, but please remember I am new to these things, so please be kind. Thanks in advance!