It's straight forward to add a giant resistive load that just converts electricity back to heat.
I can get 10kw heaters for just a couple hundred bucks or 1.5kw heaters for literally $20 usd. And that also switches on/off easily.
For hydro... just boiling water with a heater is going to be pretty much unbeatable if we're playing the "waste energy" game. No need to approximate it slowly with your pump motor and risk other infrastructure.
If you really wanted to be clever, your best bet would probably be to turn it into a laser beam and send it back to space. Would mimic that energy getting reflected instead of absorbed, albeit through a pretty inefficient and convoluted process.
Might make more sense to run carbon capture devices at that point, but again - in either of these cases we're back into the "it's getting pretty expensive to waste this energy" spot.
New designs store the power in batteries, but most locomotives used in the US still have a compartment filled with fans and resistors instead.
You can build this at a small scale pretty cheaply but the connection to sink meaningful amounts of power would quickly become a significant part of the expense.
He's just trying to burn energy because a negative rate means he's getting paid to use it.
So sure - it's great to give that energy a functional use first (ex - charge his batteries) but eventually he runs out of functional ways to use the energy but could still be making money by using it.
Enter the desire for a dummy load.