While a quick head banging can be a quick end for a smaller fish, clove oil isn't a bad death when done properly, please don't spread info otherwise. I've sadly had to use it a few times now, once on a large goldfish, and it was an easy, peaceful death each time.
The key is to add the clove oil in stages. In smaller amounts, clove oil sedates a fish - sends it to sleep. People use it to sedate large fish for operations (the king of DIY shows how he used clove oil to sedate his arawana for an eye operation, if you want to see that process on youtube) before they wake the fish up again.
To use it euthanise humanly, the trick is to add some clove oil to the container you've put the fish in, but only enough to gradually send it to sleep. Once the fish in unconscious and sedated, then you overdose the clove oil to end their suffering. The bad deaths are people who don't research how to do it humanely, put the fish in with a ton of clove oil while it's awake, and the fish panics. That's not the way to do it, but if you follow the sedate - overdose method, it's a humane way to end a fishes suffering, especially for larger fish or people who can't bring themselves to headbash, understandably.
For the OP's catfish, it's a crazy situation and I'm hoping for a good outcome! Please update us, OP. @Colin_T for your method, would you use clove oil to sedate the fish first? I'd try Colin's method first, then if that fails, the tweezers. If someone could hold a flashlight to shine down the catfishes mouth, hopefully the suction cup will be visible and grabbable...
Good luck OP, and catfish!
