Ah, yes. Building error #14A.

Couple of things come to mind if you can't get the CA to release with heat.

One, cut the horn mounting block out of the flight control surface and install a new one. Not the easiest solution, of course, if you don't have the ability to fabricate new blocks.

Two (and carefully) use a Dremel cut-off wheel to grind the heads off the screws and remove the horn. Do that carefully, so you don't chop into the flight control surface. Then grind the screw stumps even with the wood. Did I mention do it carefully ? When you mount the new double horn you can offset it as needed to locate the mounting holes over clear wood.

Which brings me to building error #14B . . . you're not really using _wood screws_ to attach control horns, are you ?

If you're not using double-sided horns it is much stronger to bolt through the flight control surface and use a backing plate. If the flight control surface is tapered, install hardwood mounting blocks and drill/tap the hardwood for the correct size machine screws.