You would like to block the local DNS usage when connected to VPN as a full tunnel? Hmm...per my understanding, Windows 10 would use both DNS servers (local & tunnel) for name resolution. If you're tunnel DNS server responds with "No such name" (RCODE 0), then Windows would fallback and use the response provided by the local DNS.
To stop this, either you need to switch to split tunnel with Device DNS only option selected or make your tunnel DNS server to respond with "name error" (RCODE 3).
I am interested to know the reason behind this particular request. can you please elaborate on the need in-hand?