We use a toenail clipper to clip our cats' claws - but that's because we have to give them medication, or they lay on our laps and knead - and it's so much less painful when their claws aren't sharp!
Each of our cats took to scratching on an appropriate scratching area and it took very little retraining. They do have different tastes in scratching stuff though, so please try different types of scratching items!
There are corrugated cardboard items designed for cats (in which you can sprinkle a little catnip). They lie on the floor. There are also scratch pads - one side has sisal, the other side is a rubbery mat. We discovered (by accident) that one of our cats LOVES the rubbery mat side of the mat that lies on the floor. He doesn't scratch on anything else.
We have cat furniture that is carpeted. All the others love to scratch on them. But we also have a sisal scratching board - it is intended to hang from a door handle, but I tied it to a wall. Two of our cats LOVE scratching on the upright sisal scratcher.
Try different types of material, different locations, try something that's vertical, something that lies on the floor, etc. You will find a combination of things that DO work. As soon as kitty understands these things are for him or her, everyone will be relieved!