Simple

Hope you have a big box of those transistors, because they won't last long with a base current like that. Try sticking a resistor (47kΩ or so) in series with the base.

The current allowed through the collector-emitter path will try to be proportional to the current passing through the base-emitter path so in this case (BJT as a switch) normally a resistor would be placed in series with the signal source and the transistor’s base. The current through the load is about equal to the beta of the transistor (very often just 100) times the current through the base. To estimate the value of the resistor for the base you can take the signal’s voltage (6V through the momentary switch) minus the base’s voltage drop (ex Vbe=0.7V) and divide that (the voltage drop across the base’s resistor) by the current you want on the load current (say 20mA) divides by the transistor’s beta. For this example it is: (6V-0.7V)/(0.02A/100)=26.5kOhm. Inserting a 26.5k resistor in between the switch and base both protects the circuit from huge current draws through the base and controls/limits the current through the load. BJTs can be pretty fickle to work with but for all intents and purposes these guidelines should let anyone design a safe BJT as a switch circuit.