You could try switching to peaceful and then back
This seems like a basic thing, but I didn't try it until after posting. I did one better and used the CoFH kill all command near my base and around my Nether portal. In both instances, it reported killing over 70 hecates. Once they despawned, spawning resumed as normal in the Nether, at least. I'll have to test the overworld later, as night hasn't fallen, I don't have much time, and my base is a little well-lit tropical island.
So apparently the problem was just an overflow of hecates that refused to despawn and in fact stayed loaded no matter where I was on a given plane. Their spawn rate is now set to zero.
I hope this means I'll be able to find the other mobs I've been looking for... I actually need Dryads, and that's the only Ars Magica 2 'object' I haven't ever located.
just on the first lava pool I came on I filled 3 drums and Im maybe halfway through this pool.
This may not be very helpful to hear, but this turned out to be a useful tip for me. I'm having similar problems: as of yet, Horizons hasn't given me any significant RF sinks, and I'm trying to Ender-Thermic Pump a decent chunk of a Nether lake for purposes of berserking at swarms of Pigmen and actually collecting Ghast drops, the problem being storage for all the lava I can't burn. Drums are actually cheaper, if uglier, than Dynamic Tanks; I just haven't ever used them before, so I didn't think of it.