Eradicating them isn't easy but it's not impossible. I've had them on a couple of occasions. One was a fairly new tank which I essentially just hit reset and drained the tank. The other instance was on an established S gigantea tank and breakdown wasn't a viable option. I wiped them out using a trap in combo with a Conus regius.
Note that the C. regius is a venomous snail that poses a threat to you as well as the worms. Extreme caution should be taken if you were to employ this option. They will hunt all annelids not just the Oenone.
Trap is pretty simple to make using PVC.
What you'll need:
1. 6-8" length by 1" PVC pipe
2. Two PVC end caps to seal pipe
3. Mesh bag to hold bait
4. Drill
5. fishing string to attach to trap(optional)
6. rubber gloves(optional)-- the worms emit a toxic mucus. I touched it once and it left a burning sensation I didn't want to repeat
Construction:
1. Attach the end caps to the pipe. Drill a 1/8" -1/4"hole in one of the end caps and one or two more just below where the end cap connects to the pipe,
2. Put bait in mesh bag and
tie bag shut. I found PE mysid worked great. Any meaty frozen should work but PE mysid is oily and really attracts them.
3. Attach fishing string to trap to aid in removal. I ran it through holes drilled in pipe.
4. Place in tank near rock work just before lights out.
5. Remove in the morning and see what you caught
6. Repeat.
Worms are incredibly light sensitive and can be almost impossible to spot using a flashlight after lights out. They can be spotted using a red filter on a flashlight. The key is to move the light slowly. Slight vibrations will also send them quickly back into the rock.
I have pics but photobucket doesn't want to cooperate
partial trap pic
mesh bag doing work
C. regius and really thick gloves