Do you have control over this server that you don't want to connect? If so, you can update the pass4SymmKey in the general stanza of the instance's server.conf, which will make it unable to successfully talk to the license master. Alternatively, you can change that setting on your License Master and every other server connected to it, but that is a lot more work and might not be worthwhile.
Another thing you can look at is creating a license pool in which you can explicitly whitelist the servers that can connect to it. While it's not as simple as blacklisting a single instance, it does give you some control within Splunk itself of what is or isn't allowed to connect to the License Master. It is also a bit more permanent than updating the pass4SymmKey on the server you don't want connecting as it prevents the same problem from happening with other rogue servers down the road.
