Depends on the make. The cheap Chinese devices are uncomfortable to wear and not very tight fitting. But higher end devices, typically handcrafted and tailor made to fit the wearer, can quite effectively block the clit. Even more effective are piercings designed specifically to afix a clit guard. These can provide genuine orgasm denial to the wearer.

My experience is with a Fancy Steel belt. I wouldn’t want to wear it long term, but it was effective. And it made me very horny.