The party keeps taking their opponent's spell casting focus away. How do I dissuade them from doing that every time?

So to be clear, I'm not upset that they've done it in the past. They decided to deal with this villain by capturing him instead of killing him, they knew exactly what his focus was and had a really good plan that worked out amazingly. Then later, after they'd gotten a bunch of information out of him, he attempted to escape, and even though that ended up being a much closer fight, they once again won by just running up to him, grabbing his focus and uhh... swallowing it so he can't get it back.

It was a cool fight and a fitting ending for this villain.

The problem is that they will fight a lot more spellcasters in the rest of this camapaign (the family of this first villain, all sorcerers) and I really don't want every fight to go like this now. So how do I prevent it? Ideas so far:

- I could try to arrange encounters so that they can't get into melee range with their opponents at all, ever. Sometimes that certainly makes sense. But there's a monk in the party and it feels a bit mean to do this all the time to be honest.

- I could make the spellcasting focus harder to grab, somehow? Thus far it was a gemstone on a piece of jewlery. Maybe the villains should get them as piercings instead lol. Anything to make it cost more than an action with a simple strength or dex check to rip it away from him, as I've ruled it so far.

- If I can't come up with anything else I'll just... ask them I guess. To prevent it from getting boring and all fights being focused on this gimmick. I'm sure they'd understand that. It just feels a bit unelegant.