Prior to vSphere Replication 8.7, all hosts visible to the VR appliance would be automatic candidates for registration to be used by VR.
When using vSphere Replication, all hosts within the vCenter Server inventory are registered automatically.
If you want to exclude hosts or a cluster of hosts under a given cluster or datacenter from registration in vSphere Replication, you can tag them with the com.vmware.vr.disallowedHost tag. This is valid for the incoming replications on the target site. The tagged hosts are still used for outgoing replications and the virtual machines on the disallowed hosts can be replicated.
Tag the host or cluster with com.vmware.vr.disallowedHost
VC:Host>Summary>Tags>Assign>Add Tag - com.vmware.vr.disallowedHost : {Desc: Indicates that a given host (or hosts under a given cluster or datacenter) should not be used by vSphere Replication }(do not rename)