How can remove activity on a single application server?
To start an application server in "passive" state, set profile parameter rdisp/start_in_maintenance_mode = 1
Another way to set an application server to "passive" is over SM51:
Goto -> Administration -> State -> Deactivate.
To change the server state back to "active", chose Activate.
You can check the server state using transaction SM51 (right column).