You do not turn off a current source by opening a switch in series. Generally a current source is turned off (or, more appropriately, diverted) by closing a switch in parallel. In the case of your circuit, however, an SPST switch cannot be used because it will also shunt the branch containing R5 (198 Ω resistor). I do not know exactly what you are trying to accomplish so I will just recommend that you replace S2 with an SPDT switch. When the switch is thrown to one position it connects to the RC circuit, at the other position it connects to ground.