I want to stop MHn server from forwarding data to Splunk.
How do I go about it so that the other forwarders in another machine are not affected?
This is basic functionality. Each universal forwarder is independent. You can stop the service all together. You can edit the config files on that forwarder to pick and choose what to/not send. You can use a deployment server to create server classes and deployment apps with client whitelists and blacklists.