How do I turn off this constant current load circuit with a Arduino gpio?
Switching off the supply is the safest option if the other opamp of 358 is not used:







simulate this circuit &ndash; Schematic created using CircuitLab

Another option is to add another IRLZ44N between the R1's lower terminal and the GND. 

PS: If you connect the load as shown in the circuit it won't work. Load's positive terminal should be connected to the positive supply rail, and its negative terminal should be connected to the drain of Q1.