Unfortunately, the thrusters won’t work very well at voltages that high. In fact, as you increase voltage they will draw more current and power. If you want to limit the current output, your best bet is to limit the signal sent to the thrusters. For instance, the normal PWM signal range is 1100-1900 µs. You can change that to 1200-1800 µs to limit the throttle output.
To see what would be an appropriate signal for the current you want, you can check the performance plots here: http://docs.bluerobotics.com/thrusters/#performance-charts
You will, of course, get a bit less thrust, but the current will be limited appropriately.
I hope that helps!