Removing broadcast for external DNS

[#1]

We have a few domains we would like to retain in the organization that currently aren't in use. Is there a way to disable broadcasting for external DNS lookup on our name servers or does the zone have to be removed completely ? I had planned to remove the SOA records and any TXT records .. etc in the zone but wasn't sure if it will still try and do a lookup. My issue is malformed SPF records when it resolves the SPF just fine, but they aren't actually being used anymore.

[#2]

Was hoping for a little bit of help here

I believe today I will remove one zone and try an SPF lookup and see what happens.