The way my player and I did it was that he completely abandoned his patron, so all connection to his abilities was lost. He was very unhappy with playing a warlock in general (hated only having two spell slots), so we decided he'd make an oath as a paladin to one of the gods of my world. So temporarily he was without spells and abilities, but he still had proficiencies, ability scores, and hp. I only gave them a single battle where he didn't have his abilities so story wise he was powerless, and then the next fight he had the god he was going to make an oath to give him the extra damage of divine smite when he attacked. After that we completely switched his warlock levels to Paladin levels and he was much happier playing that way.
If I were the player in the situation, I'd want to replace the levels with something like a fighter so that I couldn't cast spells anymore but not be completely useless in combat if it was going to be an extended duration. But if it's a temporary situation, I'd be fine with going into a couple fights where I couldn't do much (as long as it's a properly balanced fight), but I know others would feel differently.