To my knowledge there is no OOTB way to do this. An approach can be decomissioning the specific nodes from load balancer that routes browser requests. Install another load balancer that routes service requests (App requestors) to service handling nodes.
Yes, as Sayak1896 mentioned, node classification currently influences what runs on a given pega node for functionality the engine initiates (listeners, agents, QP and other background processes). For service requests each node will service a request if it arrives at that node. As such you can still use node classification alongside a properly configured load balancer so that a set of nodes only handles certain inbound service requests. Configuring node classification in that case doesn't impact runtime behavior but will likely make it easier to operate the environment.