OH! I was thinking gas turbines, not ReC. I don't know of a way to "turn of" pebble beds, doubt there is one. You can slow down fission reactors using control rods.
Your best bet is probably the higher tier electricraft batteries to keep your power-loss to a minimum.
If you want to, assuming you're just voiding your low-pressure water and not going for a closed-loop liquid system, you can potentially keep the reactor running and just let steam build up in the lines forever. Their capacity is infinite, so you can run the built-up steam through a turbine at a later time.
Onto the second thing, since this thread is both relevant and reasonably recent, Electricraft resistors are confusing.
Here's my power generation, two gas turbines junctioned and powering a generator.
Next, here's the other end of that wire, feeding into a friction heater setup. Unlimited, this motor outputs about 114 MW of shaft power. Note that there are no other motors on this wire without a resistor before them, the other 20 MW from the two gas turbines is being consumed by the ~18MW extractor setup on the left, and a couple other miscellaneous motors using less than 1MW collectively.
Now, if I try putting a 2,200 amp resistor before that motor...
...only 1,170 amps actually get to the motor.
Back at the power plant, compared to the same setup with an unresisted motor, quite a bit of power is being wasted, so I can't see any reason that 2,200 amps shouldn't be getting past that resistor.
Does anyone know if this is intended functionality, or did I hit a bug in Electricraft?