Relay circuit with off switch
This is pretty straightforward. What you need is a single pole, double throw relay with a 12 volt coil and contacts which will handle your load current. Connect it like







simulate this circuit &ndash; Schematic created using CircuitLab

When the circuit turns on, there is no power applied to the relay, so it connects the 12V to the load through the NC (normally closed) contact. 

When you connect the switch, the relay activates, and the load is disconnected. 12V is applied to the coil, so when the switch is released the relay stays activated, and it will remain so until power is removed.