How do I deal with Lava Larvae sucking all my energy?

One trick listed on the Wiki page for these creatures is to remove all power cells from the vehicle. They will detach from the hull after doing so, since there is no more energy for them to drain.

If the vehicle's energy is turned off by removing all Power Cells, or the Power Cells are depleted, they will detach.

This is a bit of a hassle, since it will require you to get out of your vehicle (excluding the Cyclops). Once you get even deeper into the lava zone, the temperature of the water will be too much for you to stand, and will begin to damage you if you don't have the proper gear. The damage is slow, but it's something to keep in mind. The Wiki page also states freezing them with your stasis rifle works. This might be an easier solution since the rifle allows for some long-range removals.

As far as any other removal tactics, you seem to already know them. Besides trying the power cell trick, there isn't much more to do. One or two larva won't drain power very much, but once enough attach, the power drain becomes fairly fast.

There are several things that have changed since I originally asked this question, and I've found some additional techniques that help manage these annoying energy leeches.

It is true that the shield generator uses up a lot of energy, but it is the most effective way to get the leeches off overall. From what I can tell, it costs about 20 energy for a single "pulse" (turning it quickly on and off again), which is about 1.6% of of the total power available if you're using power cells, or 0.33% if you're using all ion cells. If you have a bunch of larvae attached, sometimes it's better to just shock them off than to let them keep draining your battery.

Another effective way to avoid having the larvae attach themselves is surprisingly mundane - just drive faster than they can swim. As long as you keep moving forward, even at the lowest speed, the larvae can't keep up and they'll be prevented from attaching to your sub. Of course, this strategy loses some viability once you get to the inner dome where the Sea Dragons lives (I tend to move veeery slowly in their presence), but at least I don't drain all my batteries getting to them in the first place.