Implementing a constant current source circuit that can be switched off by a voltage signal
I believe the common way of making a current source would be as follows:







simulate this circuit &ndash; Schematic created using CircuitLab

The output current for the left circuit is

$$i_{out} = \frac{V_{supply}-V_{in}}{R_1}$$

For the right circuit it is

$$i_{out} = \frac{V_{in}}{R_1}$$

The components can be pretty much anything. Just make sure to simulate it once for checking stability. Current is constant as long as M1/M2 are in saturation. Switching off the current is as simple as making \$V_{in} = V_{supply}\$ (left circuit), or \$V_{in} = 0\$ (right circuit).