Often when you go to armbar someone, they grab their hands together and hold on for dear life to prevent the finish. There are many solutions, of course, but one is to use a technique called the biceps cutter.
Basically, it involves cramming your radius bone deep in your partner’s elbow crook and then twisting it. Fun stuff. It either makes them tap or let go of their hand. And if they let go, you can get that armbar you wanted.